by Carl Zimmer.
An excellent introduction to evolution.
This introduction to evolution is the companion book to the WGBH/NOVA TV series of the same name. It is very well written, and covers a broad mix of contemporary and historical issues concerning evolution.
If you're looking for a single book to give you a good background on the general subject of evolution, this is probably the book you're looking for.
Zimmer also maintains an excellent weblog, The Loom.