Of course, there were lots of books for me to choose from. In a few cases I had to choose just one book even though the author wrote several. And in a few other cases Christianity merely adapted and changed in response to a book. But at the time these works threatened Christianity to its core. You can still get them and read them for yourselves. Do so.
It's hard to judge the merits of a Thomas Paine, Bertrand Russell, and Richard Dawkins in light of these previous authors. Dawkins's book, The God Delusion, is the best selling atheist book of all time. Perhaps I should have included it, but like most other modern works of this nature, it rides on the coat-tails of these previous ones.
1) Galileo Galilei, Dialogues Concerning Two New Sciences (1632 C.E.)
2) Baruch Spinoza, A Theologico-Political Treatise (1675-76 C.E.)
3) Hermann Samuel Reimarus, Fragments (1774-1778 C.E.)
4) David Hume, Dialogues Concerning Natural Religion (1779 C.E.)
5) David F. Strauss, The Life of Jesus Critically Examined (1835-1836 C.E.)
6) Ludwig Feuerbach, The Essence of Christianity (1845 C.E.)
7) Karl Marx, The Communist Manifesto (1848 C.E.)
8) Charles Darwin, The Origin of Species (1859 C.E.)
9) Friedrich Nietzsche, The Anti-Christ (1895 C.E.)
10) Sigmund Freud, The Future of an Illusion (1927 C.E.)