I don't think that someone could write a book like the Life and Adventures of Santa Claus today. Baum wrote it in a fairy tale, folk story manner, very much honoring the idea that he was writing about children's beliefs. I think it would be hard for someone to approach the same material today without deconstructing it. And the mythology of Santa Claus is now so codified that it is practically set in stone. Baum wrote about Santa Claus before so much was defined, even though the book helped create some of those ideas, like Santa having magical assistants.
Only a few decades later, Seabury Quinn wrote Roads, an origin of Santa Claus as well as a deconstruction of Santa Claus with Santa as a Viking who served under both King Herod and Ponticus Pilate. It's an audacious book on the sheer idea alone but no one is going to accuse to of trying to create a Santa Claus that's true to the folklore.
The Life and Adventures of Santa Claus starts with being found as orphan baby by Fairies, who bring him up. After he grows up and goes out into the world, Claus learns of the suffering and sorrows of the world. Since children are the most innocent, he decides to help them by creating toys. Both as a concept and a physical object. As the scope of his toy work grows, the varies benevolent magical races help out. When Santa Claus grows old and dying, the magical races decide to grant him immortality through the power of a one-use magical item, so he can help out children for all time.
Oh, and basically behind Santa Claus's back, there is a major war between the nice enchanted races and the nasty ones who want to end his toy making, children helping ways.
I've actually read this book a number of times over the years. It's a pretty good read. While you know from the start how it's going to end (well, maybe the not immorality bit but Santa becoming Santa), it has some odd twists like that war.
And, as much as Baum's work has any kind of coherent cosmology, The Life and Adventures of Santa Claus defines it. It describes the benevolent races of fairies and ryls and knooks, as well as mentioning the original version of the land of Mo and the nomes. Baum never let continuity get in the way of a good story but this book gives us the clearest framework of his fantasy world.
The Life and Adventures of Santa Claus is a good book for understanding the work of Baum but it's also a very good children's book.