Of all the books I read - fiction and non fiction, wonderful ideas in SF and Carl Sagan - I'd have to single out Selfish Gene, as it single-handedly changed my perspective and mental/internal framework of the universe. I struggled philosophically with questions of consciousness and our place & meaning in the universe.

Note that I cannot always wholeheartedly, universally recommend all his later philosophical / atheism books to a wider audience, Selfish Gene is a no-brainer to me. --- Close second is Bad Science by Ben Goldacre.