Honestly, I would recommend not books on atheism per se, but rather ones exploring non-theistic philosophy. Schools of thought amongst atheists and agnostics are just as diverse, if not more so than amongst theists. My world view is made up of many things, humanitarianism, empiricism and so forth. Ideas that build a foundation.
There is also the more emotional side, finding beauty and purpose in life. Personally I find myself coming back to the works of Carl Sagan, particularly "The pale blue dot". Capturing, at least for me, a lot of essence.