since people have been posting non-novel stuff, i'm going to say that this is the book that has had the greatest influence upon me:
i have an older edition with nietzsche's picture on the cover. why was this the most influential book for me? it was a great cleanser-- it help me get rid of the poisons of christian and communist morality (instilled upbringing & education), and the many inner conflicts they had created. i had always been struggling with these things but this book allowed me to make a clear break. it
saved me, in this sense.
--
a warning for fools: no, nietzsche wasn't a nazi, if you think so it's because you haven't bothered to read him.