Audiobooks are stored in iTunes media file under Audiobooks, but it says they are not in my library

If my audiobooks are stored in the iTunes media file under Audiobooks but they aren't in my library when I open Music, what should I do so I can listen to them? I thought they were supposed to be accessible in the Book app. Some of my audiobooks were imported from CDs and some were purchased from Audible--maybe this changes where they're stored? I don't think I have ever purchased any from iTunes. I would prefer to get them on external drive and play them from there than keep them on my hard drive. What is the best way to do this as well?