We know by now that L. Frank Baum intended the Oz series to end in 1910 with The Emerald City of Oz. I see the next two years of his writing as him trying to re-invent himself as an author, for example the two books The Daring Twins and Phoebe Daring were released. Usually Baum would have released non-fantasy books like these under a pen name, but this time, he used his own name.
His almost-as-famous pseudonym Edith Van Dyne also did a surprising turn with the two Flying Girl books, which were a break from "her" lady-like Aunt Jane's Nieces. But I've discussed that before.
The two books from this period that people know best are The Sea Fairies and Sky Island, known as "The Trot Books." With The Scarecrow of Oz, you could call it "The Trot Trilogy." These books introduced readers to Mayre "Trot" Griffith, and Cap'n Bill Wheedles. The books were written in much the same manner as he had written the Oz books, with a difference: while the main characters visited different places, they did not return, and in each book, they were the only characters to return, almost a blank page for Baum to work with on every story.
The first book told of how Trot and Cap'n Bill visited the mermaids and destroyed the evil Zog, while the second took the two high in the air.
Sales for The Sea Fairies had not been overwhelming, so Baum decided to bring in a couple characters from the Oz series for the second adventure.
Sky Island opens with Trot meeting Button-Bright from The Road To Oz, who explains that he came to her home from his home Philadelphia by means of a Magic Umbrella. (Mary Poppins has nothing on this kid!)
One may wonder if there are other items in Button-Bright's home that may cause magical transportation, explaining how he got so lost in The Road To Oz.
Button-Bright himself seems to be much older than he was when we first met him in The Road To Oz. He is more proactive, thinks clearly, and his speech patterns have matured. Even John R. Neill's illustrations show him at least a few years older than he was in Road.
As for the Magic Umbrella, which Button-Bright says was owned by his great-great-grandfather who was an Arabian Knight (he might have meant a few more "great"s), and it's shroud of mystery about it opens up many possibilities for future stories. (Later in the story, it turns into a magical elephant.) It's almost a little sad that Baum didn't continue the Trot series instead of (or along with) the Oz Books because of all the stories that could have been told.
After proving the magic of the umbrella, Trot, Cap'n Bill, and Button-Bright decide to have a picnic on a island they've never been to they call "Sky Island," since it's just visible on the skyline. However, when they ask the umbrella to take them there, it takes them to the real Sky Island, an island in the sky!
They crash land on the Boolooroo of the Blues (literally), who rules the Blue Country. A cruel, hostile ruler, he is a little like the Wicked Witch of the West or the Nome King. His subjects obey him out of fear of being "patched," in which two people are cut in half and mismatched. He takes the Magic Umbrella, trying to figure out how it works.
In the vein of the Wicked Witch, he makes our heroes his slaves. However, they make friends. Trot, being her friendly self, makes friends with every pet of the Snubnosed Princesses, the six daughters of the Boolooroo. She finds a permanent companion in a blue parrot, who barks like a dog. Button-Bright and Cap'n Bill meet Ghip-Ghisizzle, the majordomo, who is elected to be the next Boolooroo. He informs them that the current Boolooroo has reigned his allotted 300-year reign. (The Blues live 600 years, after which they must march through the Arch of Phinis.)
Button-Bright manages to break into the vault, where he obtains the Royal Record Book for Ghip-Ghisizzle (but is unable to present it to him), but fails to find the Magic Umbrella. Trot and her friends are forced to flee to through the Fog Bank to the Pink side of Sky Island.
They discover the Fog Bank inhabited by giant amphibians, including some helpful frogs who help them through to the end. The Pinkies, they find, are more agreeable than the Blues, but Queen Tourmaline insists that custom be followed, and her council agrees to have them tossed over the edge of Sky Island.
Tourmaline is an interesting Queen. She is the poorest of the Pinks, making her seemingly their servant rather than ruler. This could be a play on how, in the Bible, as well as King, Jesus is a servant. Baum would later return to this theme in Tik-Tok of Oz with the character of Tititihoochoo.
The pinkie who casts the deciding vote is Rosalie. When she is summoned, she appears in a puff of smoke. Baum had used this before with Gwig the Mangaboo Sorceror in Dorothy and the Wizard in Oz. This was a reference to seances, which were a fad at the time. Baum was no stranger to this, as he held seances as entertainment in Aberdeen, South Dakota. (Which was, at the time, just a territory.)
While Trot and her companions are being carried to their fate, it is raining, but when they reach the edge, the rainbow comes out, and a second Oz character has an appearance: Polychrome.
Polychrome has learned a little magic since The Road to Oz, it seems, as she is able to cast a spell over a piece of clothing to keep her feet from getting wet, to make it a repeating carpet, just as Ozma's Magic Carpet had done in Ozma of Oz. Polychrome's chief purpose is to make the Pinkies look to the Book of Laws, and she reveals a law that says that the ruler of the Pinkies is the person with the lightest skin, who, Polychrome announces, is Trot.
As Queen, Trot decides to make war on the Blues, to recover the Magic Umbrella and conquer the Boolooroo. Rosalie and the Frogs of the Fog Bank help the Pinkies get to the Blue Country, where they quickly strike fear into the Blues, who retreat into their walled city, but not before capturing Cap'n Bill.
Trot uses a magic ring of invisibility to break into the Palace, where she manages to free Tiggle, a Blue who was to be patched to Ghip-Ghisizzle (who the Boolooroo suspected of taking the Record Book, but was freed by the Snubnosed Princesses, yet he escaped them to flee the city to turn himself over to the Pinkies) and finds Cap'n Bill, but he fails to escape the Blues and the Boolooroo tries to patch him to a goat the next morning. Using her invisibility, Trot manages to turn the tables on the Boolooroo and catches him in the frame where people would be cut in half for patching.
Trot declares herself Booloorooess of the Blues, and the Queen of Sky Island. She makes Ghip-Ghisizzle the Boolooroo and sets Rosalie as the new Queen of the Pinkies, who is now entitled to all the comforts her subjects have, but no more. Tiggle helps them find the Magic Umbrella, and it's home free for Trot, Cap'n Bill, and Button-Bright.
Some people see Sky Island as dealing with the subject of prejudice over skin color, since the Pinkies and the Blues cannot seem to peacefully co-exist (even at a celebratory dance, they refuse to mingle with each other) and both see the human visitors as lower forms of life, either to be used as slaves or to be put to death. It is unknown if Baum intended this to be the case.
Baum considered Sky Island to probably be his best work that he would be remembered for. However, we know that The Wonderful Wizard of Oz is the book, with The Life & Adventures of Santa Claus being a possible second. It is definitely one of his better efforts, filled with adventure, humor, intrigue, and excitement.