If you convert to mp3 at the highest standard bitrate of 320kBits/sec
you can fit 4-1/2 hours (instead of 79min 54sec)
basically the audio will take 22% as much space at this bitrate.
But audio books require nowhere near that quality of audio
(it's argueable that most music does either)
Even 160kBit/Sec might be a bit much, but 160k will allow 9hours
of audio on a single CD-R
Audio books are the one place where I think Variable bitrate is a good idea, because aside from the more dramatic readings (with different people reading different parts and occasional sound effects)
I doubt that most of the actually audio information cannot be reproduced "transparrently" from an 80kBit/Sec mp3.