Don't take this the wrong way. There's not a single book that just wins people over to atheism. JW's might believe that Satan inspires some books. It's a path of (for lack of a better term) enlightenment as one observes the comparison between belief and skepticism. For me, being soured by the God I thought I knew was the biggest help toward enlightenment. The Bible is the best book to read to learn about God and how such a maniacal overlord just cannot exist. But some others that help:
"The God Delusion" http://www.amazon.com/The-God-Delusion-Richard-Dawkins/dp/0618918248
Also by Dawkins: "The Magic of Reality: How We Know What's Really True" http://www.amazon.com/gp/product/1451675046/ref=pd_lpo_sbs_dp_ss_3?pf_rd_p=1944687562&pf_rd_s=lpo-top-stripe-1&pf_rd_t=201&pf_rd_i=0618918248&pf_rd_m=ATVPDKIKX0DER&pf_rd_r=0HV1Z9YQ97KJ1KSRYGD6
Breaking the Spell: Religion as a Natural Phenomenon http://www.amazon.com/Breaking-Spell-Religion-Natural-Phenomenon/dp/0143038338/ref=pd_sim_14_16?ie=UTF8&refRID=0HV1Z9YQ97KJ1KSRYGD6
Really, a great place to start is to follow the links at the bottom of each page of those links for "Customers Who Bought This Item Also Bought" and enjoy your own path of enlightenment and choose what you want to read.