Short takeaway: The Sea Fairies is not on par with the Oz books. Its sequel Sky Island can hold its own. The Scarecrow of Oz, as the ninth Oz novel and the third with Trot, is a letdown to both series.
Both Trot books start very differently from any of Dorothy's adventures (or Betsy's trope-xeroxing adventure). In The Sea Fairies, Trot and Cap'n Bill are going sailing near their house when some mermaids (that's what "sea fairies" means) show up and invite the humans to take a tour of their undersea kingdom.
So the first half of the book is just a random tour. It isn't even as charming as most of the Oz-related settings. Especially since so much of it is spent on pseudo-scientific explanations of how mermaids work -- none of which are actually any more sensible or satisfying than whatever "mystery" they're trying to explain.
Halfway through the story a Big Bad shows up, but everything about it that might provide some tension gets undercut. He's an evil sea monster!...who has zero power outside his own lair, to the point that he can't even lure our heroes there or send agents to kidnap them, he literally sends them an invitation and they swim right in. He keeps humans lost in shipwrecks to use as slaves!...but they're all unfazed by their state, figuring they were gonna be dead anyway, and this life isn't so bad. One of the slaves is Cap'n Bill's own long-lost brother!...who's been gone for so long that Bill barely remembers him, didn't feel much grief over his disappearance or much joy at their reunion, and there's no mention of this maybe being their cue to embark on a brother-rescuing quest.
Finally a Big Good shows up to defeat the Big Bad, and our heroes go home. The end.
It's notable that we get a lot of Trot's real-world life and family history.
Both books are more tied to real-world concerns than the Oz books get. With Dorothy, we got bits and pieces of her Kansas backstory over the various books; with Betsy, we get nothing at all. With Trot, we get a bunch of family history right off the bat.
I appreciate Cap'n Bill. He's practical, in a way that Baum's books don't often call for -- in the next book he knocks together a seat in his workshop, to make a magic umbrella into a viable flying machine for three.
And his contrariness and skepticism (not "this magic thing can't be real", but "I don't trust this magic thing to be safe")...it never pans out, everything in these books is either Obviously Good or Obviously Bad, but it's nice to see a different character type.
As for Trot...
...okay, here's the thing: Trot is mean.
Dorothy in her later appearances can be very definite about what's proper and what's not (see: insisting on renaming Bill to Billina), but with the exception of that one bizarre scene in Bunbury, she isn't rude. Trot regularly tells people to their faces how horrid they are. Look, here's her first meeting with the Big Bad from Sea Fairies:
"Well," said he, "do you not find me the most hateful creature you have ever beheld?"
The queen refrained from answering, but Trot said promptly, "We do. Nothing could be more horrider or more disgustin' than you are, it seems to me."
"Very good, very good indeed," declared the monster, lifting his lashes to flash his glowing eyes upon them.
And, sure, that guy's evil (and seems to be getting off on the insults anyway), but here she is with a completely random innocent octopus:
"Well, are we not friends, then?" asked the Octopus in an airy tone of voice.
"I think not," said the little girl. "Octopuses are horrid creatures."
"OctoPI, if you please; octoPI," said the monster with a laugh.
"I don't see any pie that pleases me," replied Trot, beginning to get angry.
"OctoPUS means one of us; two or more are called octoPI," remarked the creature, as if correcting her speech.
"I suppose a lot of you would be a whole bakery!" she said scornfully. [...]
"Let's go," said Trot. "I don't like to 'sociate with octopuses."
"OctoPI," said the creature, again correcting her.
"You're jus' as horrid whether you're puses or pies," she declared.
(In a joke that will fly right over the heads of any kid not born in the era, it turns out she's seen the contemporary political cartoons where the big trusts like Standard Oil are represented as octopi.)
I control-F'd back through the Gutenberg texts of the earlier books. Betsy never uses the word "horrid." Dorothy uses it a few times to refer to the Deadly Desert; Eureka's behavior on meeting Billina is "horrid of you"; and she uses it twice for Princess Langwidere, prompted by the fact that Langwidere first insults her and her friends, then tries to chop off her head, then locks her in a tower until she agrees to have her head chopped off. Clearly worse than "being an octopus."
Sometimes that kind of shameless certainty goes to good places. At the end of Sky Island, Trot shuts down the blue country's system of capitol punishment, and holds to it even when her new regent wants to chop just a few more people in half, come on, not even just the worst one?
But yeah, in general, she distinguishes herself from Dorothy and Betsy by being distinctively unpleasant.
Baum's writing is much-improved with book 2. It kicks off when Button-Bright drops in -- he's definitely grown, he's speaking in complete sentences now, and knows his own name and everything! -- via magic umbrella. Cap'n Bill rigs up that wooden seat, and they go for some flights, only to get stuck for a while on the eponymous Sky Island.
Apparently some kinds of magic work in the non-fairy country of America: Button-Bright's umbrella took him to a couple of different US cities before they hit the island. He and Trot use it for a morning jaunt down to the village to pick up some sewing supplies, for crying out loud.
The fantasy settings, once we get there, have the sense of fun and creativity that I expect from the Oz books. Sky Island has a pink half and a blue half, each with its own politics, customs, magical weirdnesses, and total commitment to color scheme. I was briefly excited to hear that, in the blue country, the kings are elected -- but no, when the people vote, they have to vote for whoever the king told them to. Whoops.
Meanwhile, in the pink country, the ruler is determined by...who has the lightest skin. Holy unfortunate implications, Batman.
Since this island is permanently located in the clouds, we finally get an encounter with Polychrome that doesn't involve her getting lost on the ground. And this book is officially in the Oz continuity! I wasn't sure about the relative timing, until Polychrome says cheerfully that she recognizes Button-Bright because they last saw each other in Oz.
Turns out Poly can be an ad-hoc fairy legal scholar. Neat.
By the end of the book, Trot is the new Queen of the pink country, and the Boss of the blue country. But she never considers staying -- she likes her home life in California, and she's been gone for like a week now, so her mom is probably panicking. She leaves both countries in the hands of responsible regents, and little party heads home.
The Scarecrow of Oz starts more like a traditional Oz book. Our heroes are sucked into a whirlpool, come out in a cave, and set off to find their way home.
The preface explains that this book is Baum's response to getting tons of letters imploring him to send Trot and Cap'n Bill to Oz. So, as if he's just trying to be contrary, he washes them up on the shores of Mo. And has them meet a character who chides them for not having heard of the place.
Continuity! The place still rains lemonade, snows popcorn, and has perfume-scented wind. And you can see the advancement in Baum's skill, as he doesn't just run through a list of junk-food-related weirdnesses, but reveals them naturally in response to a weather event happening, or Trot asking something.
They also bump into Button-Bright again. He's a lot more talky than in his first Oz appearance, but it feels like he's regressed from Sky Island. He's back to asking "what's x?" questions about random things that come up -- in Road To Oz, that was most of his vocabulary -- and saying rude and cranky things in general.
The Ozsolation has gone from "super dramatic" to "totally ineffectual." Our heroes bumble into Oz exactly the same way so many other outsiders have: by flying over the place and needing to land.
Technically they bumble into Jinxland, which is in an isolated and cut-off bit of Quadling Country. I completely believe the theory that Baum didn't write it to be part of Oz at all, and only retconned it in after-the-fact.
The writing in general feels like a throwback. None of the sharp wit from the last couple of books. Stock plot about an evil king trying to marry his princess niece off to an also-evil vizier, even though she loves a humble servant, who is conveniently also a prince. Lots of unsubtle punny names. The evil king is named King Krewel, fercryinoutloud.
The drama relies on the throne needing a successor, but nobody in Oz is supposed to die, so Baum has to come up with multiple awkward excuses to get rid of previous kings for good. Oh, and there are a bunch of active witches! With the excuse that they're so cut-off and so distant from the Emerald City that they can get away with it, even though they're right in Glinda's back yard, and her Magic Book means she knows exactly what they're up to.
We're in chapter 13 of 24 before the Scarecrow finally shows up.
With a bit of help from deus ex Glinda, they overthrow the evil vizier, undo some wicked witchcraft, and install the princess niece on the throne. The Scarecrow is briefly King of Jinxland before surrendering the throne to the rightful heir after a bit of relevant magic is lifted. Seems to be a talent of his.
Meanwhile: Dorothy, Betsy, and Ozma have been watching this whole adventure in the Magic Picture. It's like their very own privacy-invading reality TV.
Describes Betsy as a "shy little thing" who still isn't used to the splendor of Oz (in contrast to Dorothy, who's perfectly at home with it). She didn't seem all that shy in the last book. Nervous in strange places sometimes, but she strikes up conversations pretty easily, and rode Hank straight into the Nome King's palace without any self-consciousness. (Retroactive attempt to make them more distinct? I can dig it.)
Anyway: they decide to have Trot and Cap'n Bill stay in Oz forever.
Remember the whole cautious discussion Ozma had about whether it would be too much to extend that same invite to Betsy and Shaggy's brother, even knowing those two had no place to go? Yeah, that's no longer a thing. They don't even check whether Trot and Cap'n Bill want to live here before deciding to make the offer.
There's also a reference to Dorothy being the one who introduced Ozma to the Hungry Tiger, so, wow, Baum is just forgetting continuity right and left here.
It's such a shambles, you guys.
My impression of Trot from back when I had only read this book was "another generic not-Dorothy," and no wonder! All her distinctive characterization is limited to the first two books. She's way more generic in this one.
She's not noticeably mean. Just a little callous. (When the princess is lamenting that she'll never be able to marry the humble servant, Trot's idea of comfort is "Well, never mind; Pon isn't any great shakes, anyhow, seems to me. There are lots of other people you can love.")
She suddenly has a "grave and serious little face." That's never been a Trot thing! You know what it is? A classic Dorothy thing.
Dorothy assumes Trot will be intimidated by the glamour of the Emerald City. Trot, honored guest of the Queen of the Mermaids, herself the Queen of Sky Island! But sure enough -- in this book, she is. Intimidated by Glinda, too. The girl who had no problem insulting a murderous sea monster to its face, and now she literally needs to be handheld through meeting the sorceress.
You'd think she would've pulled rank as Queen when she and Cap'n Bill were the ones trying to intimidate King Krewel, but no.
There's the obligatory scene with the new arrivals having dinner with a bunch of Oz standbys. Trot and Cap'n Bill are astonished by the talking animals. As if they just dropped straight into Oz from America, and hadn't extensively toured two other realms with talking animals beforehand!
In the other books, these meetings are always the setup for some charming character interactions -- like Jim the Cab-Horse's huffy rivalry with the Sawhorse, or Aunt Em trying to hairy-eyeball the Cowardly Lion into submission. Here, it's just a boilerplate list of characters, and the authorial announcement that suddenly Trot feels like she's Friends With Them All.
...and then they decide to stay. On top of all the Oz-related reasons why this is weird, how does it make sense on Trot's side? She has loving parents! Who are also Cap'n Bill's dear friends. There's no struggle over how to handle that, not even a sidenote about them tragically dying offscreen. They're just...forgotten.
I can't imagine this being satisfying for all those eager letter-writing Trot fans.
Would've been a lot more satisfying as an Oz book, too, if Trot's interactions with Oz had been informed by her own unique history of fairy-country exploration. Instead, it just becomes yet another rehash of what we got in the last book with Betsy -- and this time, it isn't fresh or witty, just cheap and tired.