I enjoyed “the subtle art of not giving a fuck”. It really helped me to let go of a lot of baggage I was holding onto.

The End of Faith by Sam Harris & the Age of Reason by Thomas Paine

Guns, Germs, and Steel. Jarod Diamond.

Other exceptional literature that has affected my way of thinking is the poetry of Robert Frost snd T.S. Eliot, the songs of Bob Dylan and the anonymous text from Medieval England 'The Cloud Of Unknowing'

The Robot's Rebellion by David Icke was an eye-opener as well.

Not as an escape from religion but when I commenced really wanting to find answers to things that I didn't no the questions to. Two books I read back to back and suddenly made sense of things . In Search of Schroedinger's Cat by John Gribben and The Tao of Physics by Fritjof Capra

Although an avid reader, I didn’t really look to books when I was moving away from whatever belief I had, it was more a personal thing and about things happening in my life.

On an aside, when I saw your post on the home page what caught my eye was “Grand Island, NE” next to your pic, I also grew up in GI.

I read "Catch 22" when I was sixteen and thought it hilarious. I read it again at sixty-six and was horrified. We change continuously.

"On the origin of the Species". Nuff said.

I've read many of 'the standards'... but the two that come instantly to mind are 'the ancestors tale' by Dawkins and 'the hero: a study in tradition, myth, and drama' by Lord Raglan. By the time read them I already had 20-ish years as an atheist and both gave me several instances of "Oh shit!! I never thought of that".

Blue Highways, by William Least Heat-Moon ~

Ismael, by Daniel Quinn. I read it over when I need a tune up!

It was 30+ years ago that I was doing my questioning of faith, having been a dutiful Pentecostal in my youth. Many great freethinkers books I have enjoyed since waking up. But back then, ones that made a difference tended just to highlight some absurd assumptions of Christianity, rather than to boldly announce that they were atheist books. These included writings of Mark Twain, whom I admired as a national treasure and undeniable sharp whit. His "Letters From the Earth" was illuminating. Alice Walker's "The Color Purple," too, snuck in some challenges, when it talked about Celie's sister going off to Africa as a Christian missionary to "spread light" to "ignorant savages," only to write home after a few years to explain that she had lost her faith, the natives having opened her eyes to the ignorance of this "white man's religion."
Then there was Arthur C. Clark's Space Odyssey series, (2001, 2010, 2060, and 3001) on the surface simply sci-fi fantasy, but his books' appendices were as fascinating as the stories were and were packed full of scientific knowledge that helped me better appreciate the vastness and wonder of science and the relatively small-mindedness and inadequate, primitive paradigm of Earth-centric monotheists.
I would add Carl Satan's books, too, but honestly I didn't read any (started to read Brocca's Brain" but never finished it) but his Cosmos tv series impacted me for sure. Salute to Carl!

The Demon-Haunted World: Science as a Candle in the Dark by Carl Sagan.

All of Joe Campbell's, and his interviews with Bill Moyer

When I began questioning my religion, I searched for books to read that might provide some kind of connection with other freethinkers, past and present. I remember reading Thomas Paine's "The Age of Reason."

I was really impressed by his knowledge of the bible and the examples of biblical inconsistencies. As I read, I had my Dickson King James bible handy and read each scripture mentioned.