Two books recommended by other members here that I have read are
"Sapiens" nonfiction
"Sophie's World" fiction

Other books I have read, and can off hand recommend are
"The Handmaid's Tale"
"Brave New World"
"Elmer Gantry"
All three above are fiction.

If you are in a behavioral science field you may also want to read
"Stigma"

I have attempted to read "A History of God" by Karen Armstrong. The book is very encyclopedic and was tough for me to read. I heavily endorse "The Sins of Scripture" by John Shelby Spong, retired Episcopal bishop of Newark, NJ. I, also, found "Man's Search for Meaning" by Viktor Frankl 'non-religious' and speaks to me from the mystical side of humanity.

Hitchens: god is not great
Harris: The End of Faith
Dawkins: The Selfish Gene

Godless, by Dan Barker
God Delusion, by Richard Dawkins

Thomas Paine, The Age Of Reason
Kurt Vonnegut Jr, Cat's Cradle (or anything, really).
Orwell's 1984
Huxley's Brave New World