The Blind Watchmaker by Richard Dawkins
The book is written to convince the reader that evolution is valid.
But to me, the shocking thing was to really understand the religious argument for the first time, and understand why evolution challenges that world view.
In a nutshell, there is a web of related arguments which support the belief that God exists. One of them is that the eye is so complicated, that it must have been designed from the beginning by an intelligent being. Therefore God exists.
But Darwin showed that many small random changes plus natural selection are sufficient to explain the eye's complexity.
Why was this shocking at the time?
Just imagine you are walking around with a vague gut feeling that God must exist every time you see a beautiful bird or a flower. You figure that something intelligent must have designed that beatiful, complex living thing. You see another complex wonder of nature, and feeling gets stronger. Perhaps it becomes the main reason that you believe that God must exist.
Then one day, wham! Darwin releases his book, and it becomes clear that there is a valid scientific explanation for the complexity of that flower which does not require a supernatural designer.
Instantly your whole world view collapses. There's nothing in the science that says that God does not exist. Science only says that other explanations are sufficient. And yet, just that is enough to collapse that entire line of thinking. There are still other arguments for the existence of God. But the one you felt most strongly is gone.
Reading the book gave me a detailed understanding of that religious line of reasoning, and what it might feel like to lose it.
It gives me some understanding of why people, even today, have a desire to reject the scientific idea of evolution.