With iTunes 8.x or later you have the option to tell iTunes to treat any audio file as an audiobook. Simply select the tracks, *Get Info.*, then on the options tab set *Media kind* to
Audiobook.
Assuming, however, that the books consist of more than one file you may have problems if you plan to listen to them on an iPod. The Audiobooks feature was originally designed for books that have been stitched up into a single file and hasn't been updated to reflect the changes that now make multi-track audiobooks easy to work with in iTunes. The main issue is that the iPod lists all audiobook files by track title/chapter name so unless you rename every track using a scheme such as *Book Title, Chapter XX* the tracks will play out of sequence. There are tools for creating iPod-friendly audiobooks for which
Aldo on Audiobooks is probably the best reference. Personally I don't bother with the extra effort and just have an "Audiobooks" playlist folder containing sub-folders for each author and within these playlists for each audiobook. Although I still mark the files as Audiobooks to keep them out of my main music section when it comes to the iPod I ignore it's Audiobooks feature and use my own playlists for access.
Note that I've used leading digits in the playlist names to arrange these in chronological rather than alphabetical order.
tt2