Clip 8 GB, how much useable storage?

Anyone know how much useable storage the Sansa Clip+ 8GB has?  I started loading an audiobook series, intending to read end-to-end. 

I deleted everything else I could find on the unit first.  The books are 600-650 MB each, on MP3 CD’s.  After Book 7, I got a message when I tried to add Book 8, “insufficient memory remaining to load entire book”.

The way I do math, that is only 4.55 GB.  Can anyone explain?