iTunes has no way of knowing in that case whether the tracks are music or a book - you have to tell it.
Depending on what version of iTunes you are using, highlight all the tracks and right click, select Get Info, go to the Options tab, change the Media Kind to Book/Audiobook.
The newer versions of iTunes will then move it to Books rather than Music.
The older versions will not.
With the older versions there was a work around which I posted somewhere here years ago, but I would recommend
AudioBook Builder instead and it was $15 when I bought it instead of the $5 currently) and still recommend it for anyone that is ripping their own CDs.
The only tool of it's kind that I'm aware of and the one I used when I was still buying CDs. You can put multiple CDs into a single file with chapter markers. Just do not create single files longer than about 7 hours - you don't want to go over 8 hours, creates problems - so that 20 hr book on 30 CDs can end up being only 3 files. They have a free test drive too.
edit: Re-titled the thread also, since it's not about downloaded books, but about ripped books.