God is NOT great, by Christopher Hitchens.
The God delusion, by Richard Dawkins.
Waking up, by Sam Harris.
Forged, by Dr. Bart Ehrman.
To name a few.

To be honest, I wasn't raised in a religious houshold, so I was never a believer, and never really thought anything about it.
But as years went by, and the internet bloomed, there have been an abundance of books on the subject. I could go all day on a ton of books, but my forementioned books are at my tops of those that really make one think, or question their norm.

I started really questioning religion when I was about 8 years old or maybe even younger BUT I tend to think that actually reading the Bible, aka The Goat herders Guide to the Galaxy, word by word, verse by verse, page by page, etc, plus being expelled from EVERY Xrstian Sunday School by the time I was 9 years old for simply asking questions only served to consolidate and reinforce my rejection of religion and all of its woo-woo.

The Bible. It made absolutely no sense.

I read the bible 17 times. It convinced me it was false.

Not a book, but just listen to George Carlin talk about religion. He was a big influence on me growing up.

Mine was the Bible itself. By the time I was 15, I had read the Bible from cover to cover 3 times. Each time, I saw more and more things that did not make sense or that were simply impossible.

Joe Campbell

I became an atheist at age 13, when I realized the Bible was just a book of stories written by men.

Michigan has a hard winter that year. Bored and restless, my brother, 10, and I decided to read the World Book Encyclopedias.

Reading about philosophers Descartes and Spinoza was fascinating. In the 1700s, they were bravely anti-deity and anti-church when heretics were burned at the stake. Their rationalism theories led to the Age of Enlightenment, focusing on science instead of the Bible.

Nope. In college took philosophy, then cultural anthropology, then Old Testament and New Testament classes and somehow that combination just sealed my future as an atheist.

Oh, yes, (waving hand in the air), I can answer that one! The book that helped me transition from a child who attended church (because of my parents, mostly my mother) to someone who understood that my life would be science-based was a child's version of a Greek mythology book. Several of them, actually, including Roman mythology and Norse mythology. I quickly realized - whilst reading these stories about the gods and goddesses - that early people did not have answers to what we now know to be scientific questions, so they made up gods and goddesses with personalities, abilities, and temperaments (or quirks thereof) that explained SO much. We now understand the scientific principles to explain natural phenomena. Voilà - no more need for gods and goddesses!!

The Oxford History of Christianity. A truthful textbook that gives the honest history of how the religion was formed, what other religions it borrowed from and how it expanded, none of which caused by a god

By the time I was transitioning out of religion, I'd already been aware of several examples of how Christianity had subordinated other traditions and enforced its brand. In addition to Monty Python and Dave Allen, I would add the Inquisition, the story of Santa Clause / the Green Man, the origin of the word 'heathen', the early settling of America by Christians, and the draconian enforcement of church attendance in early Europe. The books that have resonated with me would include Sam Harris' End of Faith, Reza Aslan's Zealot: the Life and Times of Jesus of Nazareth, and Michael Schumer's Why People believe Weird Things.

I left all religion at age 12 when I started my first job .... didn't need help in leaving
Read Jonathon Livingston Seagull ....opened my mind to the possibility that with the right mindset you can achieve anything

I’ve been looking at “ The awakening of intelligence “ for the last 5 years I have about 100 pages or so left. I’m at a point to were I look at that when I get a prompting . Some of the things posed in that collection of questions took me a year or two to see .

Although I hate to admit it, Ayn Rand's "Atlas Shrugged". I read it the summer between my freshman and sophomore in college. It gave permission for intellectualism.

I no longer ascribe to Ayn Rand's philosophy. Its is flawed (politest thing I can say), and its followers subscribe to it for all the wrong reasons. Did an independent study in philosophy on Ayn Rand and her philosophy. To be young and stupid again. Lol

Anything by Bertrand Russell. And Letter to A Christian Nation by Sam Harris.

The Demon-Haunted World from Carl Sagan show how to approach the phenomena without a mystical or religious view.
To be an atheist is to continuously exercise this way of thinking and removing the shadows and demons in the corners.

One gunny book is Small Gods from Terry Pratchett, it is a massive critique on Organized religion and religion itself, Just a small spoiler, the group of atheists can only meet in buildings with copper roof because on that world gods obviously exist and smite non believers with thunders XD. This is also a critique on people that have to go loooooooong distances do justify an obvious wrong way of thinking.

The two I couldn't put down were "the God Delusion" by Dawkins and "God is Not Great" by Hitchens.

Any book by Carl Sagan.

I read the bible(KJV) cover to cover and it made me stop being a christian. I read several books that helped me become a pagan/wiccan. But, sadly no books helped me become an agnostic atheist.

The Bible.

Nathaniel Brandon’s, “The psychology of Self Esteem” and “The Blind Watchmaker” (Dawkins?).

The riddle of abiogenesis was a bit of a stumbling block for me until I read The Selfish Gene by Richard Dawkins.

The bible itself, and of course Bertrand Russell