That is hard. I don't think I can pick a single one.
The Last Hours of Ancient Sunlight, by Thom Hartman
Zen and the Art of Motorcycle Maintenance, Robert Pirsig
Witches, Erica King
Spiritual Midwifery, Ina May Gaskin
Coming of Age In the Milky Way and The Red Limit, Tim Feriss
I will stop now. There are more. 1 more is Legends of the Fall, Jim Harrison
Ok, I'm really stopping now.

The Dark Side of the Light Chasers by Debbie Ford. Talks about the "shadow self," the parts of our personality we don't see or acknowledge, and the effect this can have on our lives. Carl Jung was the first one to talk about this, but Ford's book helped me get a clearer grasp on it.

The God Delusion.

I started liking historical fiction at a young age. In 4th grade, I read "The Witch of Blackbird Pond." It dared to show children the dangers of religion. I currently own the audio book, and still listen to it occasionally.

There's been several for me .
I think the 3 that I would most recommend would be ...Howard Zinn's - The peoples history of The United States ...
Henry George - Progress and Poverty
and Derrick Jensen ...The culture of make believe ...

Grimm's Kinder und Hausmärchen; Bechstein's Deutsches Märchenbuch; Hans Christian Andersen's Fairy Tales for Children. These authors kindled my passion for words and languages.

Probably "the bible" read by many understood by few. Had a teacher who read ancient Greek and Hebrew, had access to it as it existed before king james. Modern translations all have an agenda, so did the old ones, but the take was different. What is used today is rubbish.

I would have to write the book myself!
And the book that I would want to read was very likely burnt during the "great burn" in medieval era...

The Bible and I haven't even read it.

My first book, a moral book that impacted my way of acting and being throughout my life, do not laugh now, I was around three years old and the book was Puss in Boots, I’ll going to have to read again!

Proof of Heaven , you really wonder this place of Heaven is all about.
Is it simply a white wall of nothing and freedom of non existence aka a Coma where you can’t recall all the time spent or a Castle paved in gemstones . Factorium Charles Burkowski , learning how to appreciate solitude and freedom from all the solitude Norms ( his chauvinist pig but poetic genuius in my opinion )
Abraham Lincoln’s Melancholy , Putting Depression in a new light instead of trying to constantly rid it trying to work with it to as a medium of productivity . Long Days Journey Into The Night “ An overview of Irish culture and Catholicism and despite not believing seeing how the family could turn to it giving their horrible situation despite it being the “ Norm “ then .

On religion, I don't thin there was a book. I was a not believer for a while when I read "God Delusion", but after reading it I stopped calling myself an agnostic and started calling myself an atheist. It convinced me that I was just playing up my doubt to not offend religious people as much. But I wouldn't say it has a huge impact.

I think discovering the political works of Noam Chomsky probably had the biggest impact. It was like discovering a sane outlook on the world. I spent months in libraries tracking down sources he would mention in books an articles.

Other than the Bible, since being well read in it gave me all the ammo I needed to become atheist...

The Soul of the Night by Chet Raymo, excerpt in my profile. It made sense of the beauty of the universe, and the poetry of the night sky. His essays showed throughlines between mythology, religion, literature, history and anthropology to make metaphorical sense of relativity and astrophysics and the spaces in between any two of those subjects. It was the first time I heard a rational scientist use the music of words that are often reserved for the faithful in a reverently well reasoned way. If you’re a star gazer and enjoy beautiful use of language and science, I highly recommend reading it this fall and spending some time under the night sky contemplating it. Bit heavy for a 14 year old at the time but it blew my mind.

" The Existence of God" in my freshman philosophy course in college. That was it for me

It was Foucault's Pendulum, it nearly drove me mad

Furiously Happy. I don’t read a lot. But this book made me laugh and feel normal at the same time. That was mind blowing

Novels: The Wasted Vigil by Nadeem Aslam or Housekeeping by Marilyn Robinson

Nonfiction: The Cultural Origins of Human Cogbition by Michael Tomaselli

An old book "Restoring the American Dream". That book got me acquainted with libertarian-ism.

"How to boil an egg, and other simple recipes". The only cookbook for the unenthusiastic that doesn't casually throw in unexplained bits like "saute the marinade until matured" or similar double dutch.

My 70's journals and diaries.