I have probably read a dozen books in my life and many of them affected me greatly. There are the 'screw the man' counterculture masterpieces like Catch-22 or One Flew Over the Cuckoo's Nest. Historical epics like Shogun. Horror classics like 'The Call of Cthulhu.'
But there is only one book, that every time I reread it, I e-mail the person who originally recommended it to me to thank her (so thanks again, Cindy Janes-Daily). That book is One Hundred Years of Solitude (Cien años de soledad) by Gabriel García Márquez.
It's the story of the Buendia family of Colombia. After the family patriarch kills a friend in a duel of honor, he takes his wife and some friends and heads off into the jungle, where he founds the city of Macondo. The book follows the family through many generations, from the post-colonial revolutions to the 'banana wars' to modern Latin America.
This is a magical realism novel, so in addition to the historical context, we get a lot of strange and mysterious happenings: ghosts, magic, alchemy, etc, which are accepted as normal by the characters.
To me, Macondo will always be that magical literary place where I long to escape. My Middle Earth, my Wonderland, my Hogwarts.
I know this book often appears on lists of dreary required reading, but trust me, you'll love it. Or you'll hate it, like my Philistine brother-in-law. But it was one of the few books I can honestly say touched my soul. I liked it so much that I reread it in the original Spanish. Took me almost a hundred years.