This book literally changed my life. It had me in disbelief.
I had some doubts about the church, but I still had my feet firmly on the ground in the church. After choosing to explore the doubts that I had, a friend of mine suggested I read the CES letter. I still laugh about the fact that I read it on BYU-I campus in front of everyone. I was even approached by an ex mission buddy one of the times (luckily he didn't see what it was).
After getting through it of course, there was no way I could go back. I would never have caved to anti-church documents or arguments. But facts that directly contradict teachings of the LDS church, proven using their own books? I can't believe in a god that expects me to stay in a religion that has such evident proof standing against its own claims.