Written by an former fundamentalist missionary, I'd recommend "Why I believed" to anyone who has doubts about hardline Christianity.
After considerable soul searching (and a great deal of thought and weighing up of evidence) Ken Daniels decided to resign his position as a missionary Bible translator in Africa, leave the church, and declare himself to be at least an agnostic and probably an atheist. It wasn't an easy decision because his wife, his family and many of his friends remain staunch believers. But he has no regrets…as far as he is concerned, it was the right thing to do.
This book contains his reasons why.
If this topic interests you, see also Dan Barker's excellent "Godless: how an evangelical preacher became one of America's leading atheists"
For me, this was a very influential book. Ken says (and I would agree) that he does not expect most readers to be swayed by reading the book, unless they were already entertaining doubts. I was, and he made me aware of many more things that I should be questioning, whether or not I agreed with his view on those things.
The book really has two separate parts: 1. The story of Ken's journey through life and how it led him to unbelief. I found I had many shared experiences.
2. Carefully reasoned analysis of reasons why he doubted the Bible and Christianity, as well as analysis of common objections to unbelievers.
It's a wide-ranging book, and it's impossible to remember every detail, but there were a couple of key principles I took away from it that changed the way I viewed religion: 1. Don't just read things that support your point of view.
2. Apply the same standards to evaluating your own beliefs as you do to evaluating the beliefs of others. Don't give your beliefs a free pass, whether you grew up with them or not.
In retrospect, they seem obvious, but I suspect the majority of people, like me, are drawn towards things that support their views, and away from things that might oppose them.
Finally, what Ken does is show that it's possible to leave Christianity based on an honest evaluation of the facts, not on having a vendetta against the church or wanting to live a life of sin. The path to unbelief was painful, and he did not want to follow it, but in the end he felt compelled to leave because it was the right thing to do.