I finally read the original book a few weeks ago. It was quite a quick read, but I thought it was nicely written. Some of the imagery was a bit surprising!
Before DVD's got going, I bought a 4 VHS box set of silent OZ movies. It included the 1925 silent version starring Oliver Hardy and 3 1914 silent film shorts of 3 other OZ books.
There are some other OZ books that could be considered canon. First, there are the OZ books written by Roger S. Baum, who is a great grand son of L Frank Baum. Second, the Baum Family Trust also selected Sherwood Smith to be a new "Royal Historian" of Oz and to provide new additions to the Oz canon. My problem with these books is that they do not keep the continuing plot of the other OZ books. The other canon OZ books have Dorothy eventually comes to live in OZ, while in these books she always lives in Kansas, and only visits OZ when called upon. This may seem a small matter, but if Dorothy never comes to live in OZ then all the books after book 5 has never happened. And a canon book can't radically go against other cannon books. You could say Rogers S. Baum's books are just prequels, but that doesn't work the Smith books because Dorothy isn't even even alive any more. The protagonists in those books are the descendants of Dorothy. Dorothy never could have returned to Kansas after living in OZ in a canon book, because the magic of OZ is what keeps here forever young. It is clearly said if Dorothy ever returned to OZ that she would instantly age to the time Kansas is now. That would mean she would age several decades instantly. Most non-canon OZ books you will notice leave out Dorothy and Ozma. They usually want to present an alternative OZ. Even the recent Disney Wizard movie would be hard to put in canon, because in all the plans for sequels to the movie, there is no talk of bringing Dorothy into play.
Soldier of Aslan and I have been having a discussion about the OZ books written after Baum's death. And the question of what was canonical for the OZ books came up. In CON to be canonical you just say the original 7 books written by Lewis. But for OZ books it is harder to define. Some will just say that the original 14 Baum OZ books are canon. Others will say the "famous forty" make up the canonical books of OZ. For those of who aren't familiar with the "famous forty", they include Baum's original Wizard of OZ plus the next 39 OZ books published by Reilly and Lee from 1904-1963. Those books include the 13 other Baum books, 19 by Ruth Plumly Thompson, 3 by John R. Neill, 2 by Jack Snow, and one each by Rachel Cosgrove Payes and by Eloise Jarvis McGraw and her daughter. Ruth Plumly Thompson, John R. Neill, Jack Snow, Rachel Cosgrove Payes, and Eloise Jarvis McGraw and her daughter are usually referred to as the Royal Historians of OZ by Reilly and Lee. Other books that are considered canon are other fairy books written by Baum where the main protagonists latter show up in Baum's subsequent OZ books. These would include The Sea Fairies (1911) and Sky Island (1912), which feature Trot and Captain Bill before they came to live in OZ. Last to be added to the cannon by some are those OZ books written after 1963 by any of the Royal Historians threw other publishers. But with the recent revival of OZ books, are there any recent books that could be called canonical or should they all be referred to as non-canonical?