Want to know what the fuss is all about but don't really want an anti-religious diatribe? This is your ticket. Jullian Baggini's
A Very Short Introduction to Athiesm presents the basic philosophy in a light, positive way. He's not out to convert or bash but to explain and counter some of the arguments.
Oxford's A Very Short Introduction series is an amazing collection of books on all the topics you either missed in college or want a refresher in. All of them are well-written by leaders in whichever field and include further reading.
Check them out.