I would love to write a Sources and Origins entry about Dumbo, comparing the finished film to the original story that Disney bought the film rites to, but there's one problem with that: I've never seen a copy.
Neither have you. Neither has anyone (that we know of) in many a decade. Apparently not even the Disney company has a copy in its archives.
Dumbo The Flying Elephant was not a normal book. It was one of a series of Roll-A-Books. From descriptions, they sound like book-shaped plastic containers that held the story on a scroll. Instead of flipping through the pages, you would scroll through them (it almost sounds like a super low-tech Kindle).
Michael Barrier has researched the Dumbo Roll-A-Book about as extensively as anyone can these days, and has written an in-depth and interesting piece about it here (there's no Timothy!).
I like to think that there is at least one copy of the Dumbo the Flying Elephant Roll-A-Book out there somewhere, maybe waiting in somebody's grandmother's attic. Hopefully someday, the grandkids will go clean it out and find that treasure waiting for them. They'd be instant millionaires and the rest of us would all get a look at the first version of Dumbo.