Philosophy

The Demon-Haunted World: Science as a Candle in the Dark by Carl Sagan (1995)

The God Delusion by Richard Dawkins (2006)

The End of Faith by Sam Harris (2004)

God Is Not Great: How Religion Poisons Everything by Christopher Hitchens 

Collected Writings by Thomas Paine (pub. from 1776 to 1806) starting with “The Age of Reason.”

Letter to a Christian Nation by Sam Harris

The Works of Robert Green Ingersoll and here they are: A list of works

Might we also suggest:
 

The Incredible Unlikeliness of Being by Professor Alice Roberts

Grandmother Fish by Jonathan Tweet, illustrated by Karen Lewis

Why I Am Not a Christian by Bertrand Russell

Why I am not a Muslim  by Ibn Warraq

An Elegant Description of Reality by H. Dabbouk and K. Mohsen

A Treatise of Human Nature and Dialogues Concerning Natural Religion by David Hume

and, also by David Hume, Dialogues Concerning Natural Religion