Books Of Wonder including: The Amber Flute Of Oz, Father Goose In Oz, and The Speckled Rose Of Oz. He even decided to do his own prequal to the original Wizard Of Oz, a few years before Gregory Maquire had started his Wicked series. Abbott started what was essentially a two-part prequal with How The Wizard Came To Oz.
The Wizard's secret origins are a little similar to most of what was referred to in Oz books. Oscar Diggs is a circus performer who learns various tricks of the trade, and one day takes the circus' balloon for a joyride, but ends up in Oz. He is confronted by the Wicked Witch of the West who runs the Winkie Country. She summons up her allies: the wolves, the bees, and the crow, but Oscar uses his ventriloquism skills to scare them all off. The Winkies proclaim him to be the Wonderful Wizard of Oz, and the Witch heads east to seek help from her sister. The Witch of the East tells her sister of the magical Golden Cap which she can use to control the flying monkeys. The Wicked Witch eventually claims the cap, and forces the monkeys to drive the Wizard out of the west by balloon. After landing in the north of Oz, the Wizard manages to defeat a group of Kalidahs. The grateful citizens then help him begin constuction on the Emerald City, to which the Wizard uses the trick of the green spectacles to make it seem greener than it was. The Witches then join forces to launch attacks on the Emerald City. The East Witch employs the silver shoes' magic to create a yellow brick road straight to the city for her troops, as well as a giant spider(the one the Lion later fights). The Wizard manages to ward them off with more of his circus tricks, and saves the Emerald City...for now.
This book leads into the sequal(but still prequal)How The Wizard Saved Oz. It acts as a pretty good prelude for the Oz legacy, although it seems to skip through alot of the more specific details of the Wizard's early years. There's even some lead in to the Scarecrow and Tin Woodman's origins. Donald Abbott has some slightly impressive illustrations too as he modeled his work after W.W. Denslow. If you want to see a comprehensive and enjoyable view at the Young Oscar Diggs Chronicles, then give this one a look.