My discovery, using Windows computer and iTunes 12:
I transfer mp3 audiobook files by dragging them (using Windows Explorer) into a playlist in iTune (I create an appropriately named playlist first). In iTunes I then select all the files in the playlist (Ctrl+A), right-click and select 'get info'. On the 'Options' tab I change the 'media kind' to Audiobook. If all the files have the same album name, they appear in 'Books' and 'My Audiobooks' as an audiobook in the iTune list. I then sync the book to my iPhone.
Hint: I use a free program called Tagscan to modify the mp3 files before importing them to iTunes. As iTunes ignores file names and uses the tags for file identification, I use the Tag Processor tab in Tagscan to change the tags to be the same as the file names). I also use Tagscan to give all the files the album name using the name of the book (which becomes the name of the audiobook) and a cover picture (just for fun).
The problem I came across recently is that I can change the media kind for multiple files, but if I have an audiobook that comprises only one mp3 file, iTunes refuses the change the media kind. The trick is: I put another mp3 file in the same playlist, select both files and change the media kind to audiobook, then delete the second file.