CoC - Raymond Franz - No need to explain why.
The God Delusion - Richard Dawkins - I at least began to understand atheism from reading this back in 2006.
God and the Folly of Faith - Victor J. Stenger - Put any question of belief in the unprovable behind me.
Fabric of the Cosmos - Brian Greene - Best overall description of relativity and quantum mechanics I have read. (The string theory parts can be safely ignored since the discovery of the Higgs boson).
Thinking Fast and Slow - Daniel Kaheman - Still reading this, but the overall best insights into human psychology I have read.
Fooled by Randomness - Nassim Nicholas Taleb - Helped me to understand how counterintuitive probability plays a significant role in outcomes in the universe and our lives.
Einstien, (Bio) - Walter Issacson - Lost my balance, (literally), when I understood general relativity for the first time while reading.
d4g