Hello,
I had the same problem... on my iPhone ibooks app my audiobook did not appear (but all the ebooks do). I downloaded a third-party app called Bookplayer. It does the job. I can create a folder for the book title and put all the tracks in it.
In my iCloud drive I created a folder for Audiobooks, then sub-folders for each title. When I add them to Bookplayer, I can create a playlist for my audiobook title. It works well for me and I can move tracks around within a playlist to change the order of play.
I bought a new desktop computer this summer with the latest OS and I've been very disappointed how iTunes is no longer available. I haven't found the Music app to be user friendly and the fact we can't play our audiobooks on ibooks is ridiculous. BUT, Apple's Book Store is everywhere, can't escape that.