It often seems to be the case that people who read Oz books by authors other than L. Frank Baum when they were young accept them as official, while those who read them later are a little more wary of them. Back in the day, they were marketed as all part of the same series. After Baum died, the publishers asked Ruth Plumly Thompson to continue the series, and Baum’s widow approved. Her first Oz book, The Royal Book of Oz, was even credited to Baum, with Thompson said to have “enlarged and edited” it. Actually, it’s very clearly Thompson’s work throughout, but the publishers apparently wanted to ease the transition. I’m sure they mainly wanted more Oz books for the money, but if Baum had seriously objected to someone else continuing the series, I kind of doubt Maud would have approved. Then again, she does seem to have worn the pants in the Baum household. Four other authors wrote Oz books for the same publisher, and libraries often just sort all of them under “Baum.” It certainly makes them easier to find. I’d heard that other writers had continued the series after Baum before reading any Oz books, and it turned out that the fourth one I read was a Thompson. It wasn’t even one of her better ones, but it didn’t occur to me not to accept it as part of the same continuity. I think people who care about Oz canon at all generally accept at least the books that have come to be known as the Famous Forty, also throwing in the Little Wizard Stories and some other Baum fantasies that are tied into the series. There are others, however, who are quite adamant about only accepting Baum. There’s certainly some merit to the argument, as he was the one who created the franchise, and his successors tended to take it in directions he most likely wouldn’t have. Personally, I accept not only the FF, but others as well.
Robert Pattrick’s essays on Oz, collected in Unexplored Territory in Oz, were groundbreaking in terms of research on this fictional place. One of these, “Oz vs. Authors,” dealt with the question as to which books should be accepted as official, with a clear bias toward an inclusive attitude. As the annotations by Patrick Maund mention, Pattrick doesn’t completely distinguish between whether a book is worth reading and whether it’s considered canonical. There’s also the question as to what you’re doing with your take on the issue. Some people just read, some write their own continuations, and some analyze the stories as if they’re real. Pattrick states that James Thurber only accepts the first two books, which is true. Thurber wrote an essay in which he makes clear that he sees just Wizard and Land as authentic and worth reading. But then, Thurber wasn’t writing his own Oz stories or Oz-as-history essays. Some fans write material based only on Baum, which is fine, but it does come off as a little arrogant when they dismiss other authors who were doing the same thing they are. Since none of the official post-Baum books really changed Oz all that much, though, a continuation based only on Baum isn’t necessarily going to contradict them. There’s a difference between not using Thompson as a source and purposely going against what she wrote. It’s often not really an issue, because the post-Baum authors didn’t change all that much, and even when they did there’s often a fairly easy fix for contradictions. That’s if they use all of Baum as a baseline, anyway; tales that branch off from just Wizard or just the first six Baums tend to be irreconcilable with the rest of the series. Oddly, I’ve seen a few works that consider only the first SEVEN Baums, which strikes me as a strange cut-off. Baum originally intended to stop with the sixth book, but once he wrote the seventh, he was in it for the rest of his life. I’m not sure it matters a whole lot to most fans anyway, but it does if you’re trying to get a coherent picture of the history of Oz. Pattrick claims that people who go beyond Baum are more interested in Oz as a concept, which isn’t necessarily true, but there’s probably some merit to it. After all, a later author might have expanded upon an issue or solved a mystery that you’re researching for your own work. And if you’re working from the conceit of Oz as a real place, wouldn’t it make sense to consult multiple sources instead of just one?
Going beyond the FF authors is a more difficult matter. One of the most inclusive lists of books is Joe Bongiorno’s Royal Timeline of Oz, but there are things I accept and he doesn’t, and vice versa. Since Dick Martin’s Ozmapolitan seems to be considered at least deuterocanonical as it was written by someone who illustrated one of the FF, it would also make sense to count the work of Eric Shanower, who illustrated three books by FF authors that were published later (Rachel Cosgrove Payes’s Wicked Witch, John R. Neill’s Runaway, and Eloise Jarvis McGraw’s Rundelstone). Beyond that, it’s a little more up in the air. One of the more inclusive lists of Oz books is Joe Bongiorno’s Royal Timeline of Oz, although there are some stories I accept and he doesn’t, and vice versa. His Supreme Seventy-Five includes the FF and other works by those tied to the FF, as well as a few others that are significant in establishing the history of Oz. Some of these are by authors who have no direct connection to any of the FF authors, and published by small presses in limited runs. I try not to judge too much on the quality of the stories, although obviously if they’re awful, I’m probably not going to pay enough attention to be concerned with fitting them into continuity. But there are, for instance, some books written by children that were published with very little editing. While the ideas in them are generally good, they often come across as sloppy in some respects. It’s not just stories by kids that come across this way, but they make for good examples. I’ll usually accept them as accurate if they don’t mess too much with established continuity, but might take them as somewhat less than faithful in their descriptions of particular actions and dialogue. Of course, even if you ARE using post-FF books are sources, there’s a good chance you wouldn’t have read all of them, which can result in unintentional contradictions. The old Nome King’s story has gotten really convoluted over the years, with a few different books ending with his reformation, and others making him even meaner than he originally was. The Royal Timeline ties a lot of them together, but there are a few it doesn’t include for various reasons. Mrs. Yoop’s history branches off in different but not necessarily mutually exclusive ways as well. When I write Oz stuff, I’ll reference non-FF material, but usually not in ways that I think impact the story all that much. But then, I’ll also reference events that haven’t been written about at all, because I assume things keep happening in Oz even when authors aren’t recording them.