Recommend introductory books about Atheism/Agnosticism/Existentia…

My own beliefs teeter somewhere between Atheism,Agnosticism,Existentialism, and Nihilism.So I am looking for introductory type books dealing with this subject matter.So I can better define my own beliefs. I haven't really read any literature as far as Sagan etc goes either.So any titles or things like that would be helpful.