The History of the Oz Books

The son of a wealthy oil man, Lyman Frank Baum grew up indulged by mid-19th-century standards on his family's New York estate. He had a weak heart and could not enjoy many of the athletic activities of young boys. At the age of 12, his family sent him to Peekskill Military Academy. Army life did not agree with him and, after two years, he suffered a heart seizure which sent him home.
Free again to pursue his interests, he and his brother, Henry Clay, published a newspaper called The Rose Lawn Home Journal, using a printing press which their father bought for Baum at the age of 15. It included short stories, poems, riddles, scientific, and nonfiction articles. Two years later, with help from a friend named Thomas G. Alford, they started their second paper, The Empire. This featured, among other things, news about postage stamps.
During Baum's early adulthood, he explored a number of career fields, including acting, theater management, and chicken breeding.
In 1882, he married Maud Gage and, over the next six years, had two sons. Like several authors of children's books, this gave him a chance to exercise his fertile imagination and he entertained them with stories.
The first of several financial difficulties came to head in 1887, when his father died. A clerk had embezzled most of the oil company's capital, leaving it dry. In an attempt to put this setback behind him, he moved to Aberdeen, South Carolina and opened a bazaar. It was foreclosed on two years later, mostly due to Baum's generosity in refusing to accept money from destitute customers. His third newspaper also collapsed due to the firmly established competition.
With two more sons, he moved on to Chicago in 1891, where he found work as a reporter, a department store window dresser, and a traveling chinaware salesman. In 1896, he applied for copyright on his first two children's books, Adventures in Phunniland and Tales from Mother Goose. The Mother Goose book was published in 1897 under the name of Mother Goose in Prose, a collection of short stories that featured a little farm girl named Dorothy. Encouraged by the success of this book, he then began producing The Show Window, a magazine for window dressers which was an immediate success.
Teaming up with artist William Wallace Denslow, he turned out Father Goose: His Book in 1899 and sold 60,000 copies. This would soon be overshadowed by this team's next effort, titled The Wonderful Wizard of Oz, which was released at the turn of the century. The general public loved it and at least one critic compared it to Alice in Wonderland.
Though Baum kept a scrapbook of critics' reviews of The Wizard, only two of which were unfavorable, he did not fully realize the power of this book and turned his attention to other books. In 1904, though, he released The Marvelous Land of Oz to help stem off financial difficulties. In 1907, Ozma of Oz was released and, after that, he produced one Oz book a year until The Emerald City of Oz, which he announced would be the last.
A combination of financial difficulties, which included bankruptcy, and children's please for "more Oz" and "more Dorothy" caused him to capitulate and The Patchwork Girl of Oz was released in 1913. He was dubbed "The Royal Historian of Oz" and continued to release an Oz book every year until his death in 1919. The last book by him, Glinda of Oz, was published posthumously in 1920.

How many Oz books are there? L. Frank Baum wrote 14 full-length children's books and one collection of short stories titled "Little Wizard Stories of Oz". The Baum books are, in order of publication, "The Wonderful Wizard of Oz," "The Marvelous Land of Oz," "Ozma of Oz," "Dorothy and the Wizard of Oz," "The Road to Oz," "The Emerald City of Oz," "The Patchwork Girl of Oz," "Tik-Tok of Oz," The Scarecrow of Oz," "Rinkitink in Oz," "The Lost Princess of Oz", "The Tin Woodman of Oz," "The Magic of Oz," and "Glinda of Oz."

"I was out on a California beach," said L. Frank Baum. "There was a pretty little girl I was very much interested in beside me. She was making sand pies and enjoying herself immensely, when suddenly she saw one of those little sand crabs, fiddler crabs I suppose they are.

'Oh, what is it?' she said.

'A wogglebug,' I said, unthinkingly, using the first term that popped into my head.

'The child was delighted and ran to her parents shouting: 'Oh, see what I've got! It's a wogglebug. Mr. Baum says it's a wogglebug.'

"The name was so catchy that the same evening my wife told me I should put the Wogglebug in 'The Marvelous Land of Oz.' The book was one-third written and Jack Pumpkinhead was the hero, but I brought in the Wogglebug right away.

"After that H.M. Wogglebug T.E. was the hero and has become my most popular character."

After Baum's death, author Ruth Plumly Thompson took up the role of "Royal Historian of Oz" and wrote 19 Oz books. Her books are more in the style of fairy tales than Baum, and she also changed the spelling of "nome" to "gnome."
"The Royal Book of Oz" has been mistakingly credited to Baum, though Thompson is the sole author. The Scarecrow, searching for his family tree, discovers that he is the lost Emperor of the Silver Islands.
"Kabumpo in Oz" introduces new visitors to the Land of Oz, Prince Pompadore of Pumperdink and Kabumpo the Elephant. Pompadore must marry a "proper princess" to save his kingdom, so he journeys to the Emerald City and helps save Oz from the Gnome King. This is the first appearance of Thompson's corrected spelling of "Gnome."
"The Cowardly Lion of Oz" finds himself kidnapped by Mustafa of Mudge to add to his menagarie of lions.
"Grampa of Oz", a former sailor, and Prince Tatters of Ragbag aim to find King Fumbo's lost head and a treasure to save their kingdom.
"The Lost King of Oz" brings back Old Mombi, who has not been seen since "The Marvelous Land of Oz." An encounter with the former Prime Minister of Oz sends her off to find the long-lost King Pastoria.
"The Hungry Tiger of Oz" becomes a victim of his own appetite when the king of Rash lures him away from Oz. There, he discovers that a few friends are prisoners and helps them escape.
"The Gnome King of Oz", Oz's longtime nemesis, finally gets his own book! Peter Brown finds his way to Ruggedo's island and helps him escape to continue his plans to conquer Oz.
"The Giant Horse of Oz" introduces another of Mombi's evil spells. The royal family of Munchkinland are kept prisoner by Mombi's evil monster, who now wants a mortal maiden.
"Jack Pumpkinhead of Oz" makes a wrong turn on a trip to the Emerald City and ends up in Quadling Country, where he discovers a plot to conquer Oz and marry Ozma.
"The Yellow Knight of Oz", Sir Hokus of Pokes, grows bored with life in the Emerald City, so he leaves in search of adventure. He eventually discovers that his memories of "Merrie Olde England" are false.
"Pirates in Oz" brings back Peter and the Gnome King, this time involving Captain Samuel Salt and his band of pirates.
"The Purple Prince of Oz" gets in trouble when he claims the king's grapes are sour. Kabumpo the elephant steps in to make the Purple Prince his attendant and, later, they must save the day when the royal family disappears.
"Ojo in Oz", who was introduced in The Patchwork Girl, gets kidnapped by Gypsies, escapes with fellow prisoner Realbad, and visits Crystal City and Unicorners on the way home.
"Speedy in Oz" is inspecting a dinosaur skeleton when a geyser blows him in the air. The skeleton comes to life and they land in the floating Umbrella Island.
"The WIshing Horse of Oz" starts in Skampavia where King Skamperoo wishes for a horse using enchanted emerald necklaces. When Chalk, the Wishing Horse of Oz, falls from the sky, Skamperoo decides the emeralds must be from the Emerald City, and decides to conquer all of Oz.
"Captain Salt in Oz" sails the Nonestic Ocean and discovers Ozamaland, a legendary land of flying animals, as well as the famous White City of Om and many other places.
"Handy Mandy in Oz" is a Mernite, a race of seven-handed people. One day, while Mandy is trying to gather her goats, the rock she is standing on is blown into the air and into Oz. She lands in Keretaria in the Munchkin Country and meets Nox the white Royal Ox. This is also the last appearance of Ruggedo, the Gnome King.
"The Silver Princess in Oz", Planetty from Anuther Planet, teams up with King Randy of Regalia, Kabumpo the Elephant, and the fire-breathing Thundercolt for a series of adventures.
"Ozoplaning with the Wizard of Oz" was the last by Ruth Plumly Thompson and released in 1939, the same year as the movie "The Wizard of Oz." It brings back several old friends from the original Oz books as they go off on adventures in the Wizard's experimental Ozoplanes.

Oz illustrator John R. Neill took over the Oz series after Thompson's retirement. His three books, "The Wonder City of Oz", "The Scalawagons of Oz", and "Lucky Bucky in Oz", tended to be more manic than Baum's or Thompson's renditions. He took each Oz region's color themes (red for the Quadlings, blue for the Munchkins) to extremes, even extending it to sky and skin color.
Jack Snow was an Oz scholar who, at the age of 12, offered to take over the Oz series after Baum's death. He did not include any of Thompson's or Neill's characters in his own books, though he did create his own. His two are "The Magical Mimics in Oz" and "The Shaggy Man of Oz".
Rachel R. Cosgrove only wrote one book, "The Hidden Valley of Oz," in which Jam, a boy from Ohio, builds a kite and attaches it to a crate and sets off to Oz with his two guinea pigs, Pinny and Gig, and a lab rat named Percy. Once in Oz, Jam realizes his pets can talk. He lands in the Hidden Valley and becomes a prisoner, but they escape and set out on adventures with the Tin Woodman.
Eloise Jarvis McGraw and Lauren Lynn McGraw co-wrote "Merry Go Round in Oz", in which Robin Brown from the USA rides a magic merry-go-round horse named Merry Go Round to Oz. Upon landing, Robin must help find the missing magic Circlets of Halidom. This is the last of the "Big Forty" that makes up the official canon of Oz.

Instead of boring you with "proper" source format, I'll just post a few links where I got my info and you can find out more about L. Frank Baum and the Oz books.

Wikipedia:
L. Frank Baum
List of Oz Books

Also see:
Oz Central
Interview with L. Frank Baum