L. Frank Baum introduced that book at the end of The Emerald City of Oz. It records everything that happens in the world—with very little detail, naturally. Glinda reads the book regularly to keep watch on threats to Oz.
Sometimes the Book of Records is useful for setting plots in motion, as when Dorothy and Ozma read about a war between the Skeezers and Flatheads in Glinda of Oz and decide to go put things right. At other times the book provides a quick explanation for a necessary plot twist, as in Tik-Tok of Oz. But most often it, like Ozma’s Magic Picture, is an obstacle to building a suspenseful plot. Since the rulers of Oz can very quickly learn what’s going on, how can anyone threaten Oz? How can anybody from the Emerald City get into trouble?
Despite all he and his successors wrote about the Great Book of Records, Baum never explained how it worked (besides “magic”) and how Glinda came to possess it. In The Marvelous Land of Oz, Glinda tells the Scarecrow that she has a book with lots of facts about the Wizard, of the same level of triviality and limited helpfulness that the Book of Records is later shown to provide. But the sorceress says she came by that information from spies.
The Ozopolis theory ties the Great Book of Records to the beautiful young women whom Glinda recruits as her handmaidens. Baum mentioned them multiple times, always describing them doing ordinary genteel handwork.
Kushin’s artist, Gonzalo Martinez, instead shows a couple dozen seated in front of a giant crystal, blindfolded and writing with quills in small notebooks. “The same thaumaturgical engine that powers Ozma’s Magic Picture enhances the scope of my maidens’ visions,” says Glinda. What they write in those notebooks gets magically transcribed into the Great Book. This system “was created soon after Ozma’s coronation,” Glinda says, and the passive voice doesn’t conceal that she herself seems to be the driving force behind the creation.
I have a different theory, but I view Glinda as practicing more realpolitik and secrecy than some like. I posit that the fairy queen Lurline created the Great Book of Records for Ozma’s adoptive father Pastoria, king of Oz. He wasn’t able to keep up with its information before being kidnapped and enchanted. Glinda took possession of the book for the sake of the country, at first concealing its existence through her cover story about the spies. After Glinda helped Ozma ascend to the throne, Lurline gave the new little queen the Magic Picture and Glinda got to keep the book. As for how it actually works, I go back to “magic” and let the handmaidens tend to their embroidery.
There are no doubt other theories to explain the Great Book of Records. In fact, there’s probably a one-to-one correlation between Oz fans and theories about Oz’s mysteries.