Trot and Cap'n Bill had actually been introduced in 1911 in the book The Sea Fairies. Mayre "Trot" Griffiths lives with her mother, who boards Bill Wheedles, a retired peg-legged sailor who cares for Trot, serving as a surrogate father as her actual father is often away at sea. Trot is a curious but strongly opinionated little girl, while Cap'n Bill is a grizzled old sun-tanned sailor who fears only what he can't defend Trot from.
The two go exploring together, and the plot of The Sea Fairies finds them being turned into mer-people so they can visit with the mermaids and the sea creatures. They become friends with the mermaids and King Anko, the great Sea Serpent. Trot is even given a ring to summon the mermaids when she needs help near the water.
To attempt to boost sales, Baum's much more substantial sequel, Sky Island, brought in Button-Bright and Polychrome from The Road to Oz, tying the series to the Oz books. Button-Bright arrives via a flying Magic Umbrella to Trot and Cap'n Bill's home. He visits with them and shows them how the Umbrella works. By accident, it takes them to an island in the Sky, where they are made prisoner by the Boolooroo of the Blues. Finally, they escape to the Pink side of the Island, where they are almost executed until Polychrome arrives in the nick of time to save them, using a law that makes Trot the new queen of the Pinkies. With the help of the Pinkies and a little magic, the Blues are conquered, making Trot the "boss." After recovering the Umbrella, Trot, Cap'n Bill and Button-Bright return to Trot's home in California.
There's some debate as to whether or not The Scarecrow of Oz was a revised third Trot book. Oz proper doesn't appear in the story for several chapters, but the book finds Trot and Cap'n Bill being sent underground by a whirlpool. If The Sea Fairies represented the element of water, and Sky Island represented the element of air, did Scarecrow represent the element of earth?
Yet a counter argument rises in that The Sea Fairies and Sky Island take their time to get the story started, while Scarecrow begins with Trot and Cap'n Bill rowing out into the water and being caught in a whirlpool. Yet it is possible that Baum revised the opening and chopped off an opening chapter or two.
Trot then becomes the third girl from America to live with Ozma in the palace, joining Dorothy and the Wizard's search party to find Ozma in The Lost Princess of Oz. However, she and Cap'n Bill have another adventure in The Magic of Oz as they try to get a magic flower for Ozma's birthday. They have a couple close shaves on the way, and need the Wizard's help to finish the task.
John R. Neill was a little loose about Trot's hair color. She almost looks like Dorothy in some Scarecrow pictures, but seemingly, the generally accepted hair colors by fans of the books are that Dorothy is blonde, Ozma is a brunette, Betsy has auburn hair, and Trot has black. (Remember, generally accepted. Anyone wishing to change this up for their own work or ideas is welcome to.)
Trot and Cap'n Bill generally play minor roles in the rest of the Famous Forty Oz books. Cap'n Bill makes Trot a wooden doll from a tree that princess Peg Amy was transformed into, which is stolen, enlarged, and brought to life by Ruggedo in Kabumpo in Oz, before she is restored to her original form. In The Giant Horse of Oz, looking for a maiden to satisfy Quiberon, Trot is kidnapped by Akbad. She escapes with Benny and the Scarecrow, who were carried away. In the Famous Forty + book The Wicked Witch of Oz, the Witch Singra accidentally turns Trot into a piece of cheese when she meant to do it to Dorothy. (She is, of course, restored by story's end.)
Outside of the Famous Forty, Trot has had more adventures. In the late Marc Haas' The Medicine Man of Oz, many of the Giant Horse crew reunites for a new adventure, including Trot. Eric Shanower originally conceived The Enchanted Apples of Oz as a Trot and Cap'n Bill tale, and later cowrote Trot of Oz with Glenn Ingersoll. Finally, David Tai offers a Trot who is a little more impetuous than her Famous Forty appearances to give her a more distinctive personality. So far, David's only published story with Trot is in Oziana 2008 in "Executive Decisions." He's written more stories that await publication, however, many of which have his take on Betsy and Trot bouncing off of each other.
Scans from The Scarecrow of Oz courtesy Marcus Mebes