Lukeprog at commonsenseatheism.com posted on his blog about a book called "The Testament", which is a series of letters written in secret by a French Catholic priest named Jean Meslier, who lived in the late 17th and early 18th centuries.  Meslier was afraid of being branded a heretic and killed or tortured for his views, so he secretly wrote these letters for his friends and family to find after his death.  It is likely the first book-length essay promoting atheism.

Here is the wikipedia entry about him:

http://en.wikipedia.org/wiki/Jean_Meslier

And here are the Common Sense Atheism blog entries about his work:

http://commonsenseatheism.com/?p=7635

BTW, the first English translation of his work was released last year and is available on sites like Amazon or Barnes and Noble.

Among the chapters include:

3.  All religions are nothing but errors, illusion, and imposture

5.  Reasons why politicians use the errors and abuses of religion

14.  Uncertainty of the so-called Holy Scriptures

17.  Contradictions of the Gospels

18.  The miracles reported are unbelievable and the reason why

20.  Neither one is more credible than the other

32.  What was the mind and character of Jesus Christ?

34   Christianity was only a vile and despicable fanaticism in the beginning

40.  Three principle errors of Christian morality

61.  Where do the first belief and knowledge of the gods come from?

70.  Creation is impossible and nothing can ever have been created

71.  Being and matter, which are the same thing, can only have its existence and movement from itself

76.  There are many false prophets and many false miracles

93.  Pliny, the famous naturalist, did not believe the sentiments on the subject

This book was written between the years 1719 and 1729.  This is over 130 years before Darwin published On the Origin of Species and over 200 years before the Big Bang was first proposed.

I can't believe that such a revolutionary work, which heavily influenced Voltaire, has not become more well-known until now.  This guy was centuries ahead of his time.