Select the albums/songs and press Command+I to call up the Info window. Choose Options, and pick Audiobooks in Media Kind. The albums/songs you choose will show up in the Book section of your library.
[crarko adds: I tested this, and it works as described. I suppose this could come in useful if you do audio recording and want to distribute it as an Audiobook instead of a Podcast. Note that you can also classify files as Podcasts, iTunes U, Voice Memo, or simply Music using this method.]

