The book that meant the most to me would probably be Boccaccio's Decameron. It was the only book that I actually read from cover to cover in Lit-Hum (which was also the only time I read more than what was assigned). Years later, a girlfriend and I would read to each other from the same dog-eared copy on a deserted beach in Costa Rica. I suppose we imagined ourselves wating out a plague as well.

Next in line would be Kant's Groundwork. It was the first time I've ever had a philosopher actually read my mind hundreds of years before I was even born, and put my intuitions in a more detailed form than I ever could.