I have the following audiobooks in the latest iTunes under Windows 10:
I have imported several audiobooks from Audible into iTunes. Each book appears under "Audiobooks" on my iPod Classic as a single entry.
I have also imported several books purchased elsewhere, as individual MP3 files. I have ensured that the title, author, etc. are identical and have numbered all tracks correctly. I have set the Media Kind to "Audiobooks" and have selected the "part of a compilation" option as well as the "skip when shuffling" and "remember position" options. In iTunes, these books appear correctly, each as one entry, with all the tracks in the correct order.
I have also imported one original CD audiobook (11 disks) and have curated the meta-data as described above. Again, in iTunes, the book appears correctly.
After syncing to my iPod Classic, all the Audiobooks appear, but only the Audible ones appear as a single entry per book. Each track of each of the MP3 books and the CD book appear as individual entries/books. This makes for a very long list, and, of course, makes it very inconvenient to listen to. I would have to scroll to "Chapter 1" of the book I want, listen to it, and then scroll to "Chapter 2" of the same book, etc. "Chapter 1", "Chapter 2", etc. for each book appears in the audiobook list.
I have searched extensively, but have not found a solution. This answer did not solve the problem I describe.