I regularly use AG for Audio books using Lame codec and an 8 CD book will be about 250 mb or 8 books to your player.
Don't worry about the quaity compared to a music file, it is not the same!
The mp3 Settings I use are 1/. Lame as Internal Encoder with a 2/. Contant Bitrate of 32 with 3/. the Quality set to Joint Stereo and Voice. These settings are perfectly OK for the usual single narrator Audio Book.