Any book worth reading is worth re-reading. Why? Because each time you revisit a book, or any other work of art, you'll take something new away from it. You'll likely have a much different interpretation of a book you read at eighteen if you were to read it again at twenty-seven, or thirty-six, or fifty-two, etc.
It's too difficult for me to choose a single book that's changed my life, so here are several:
"The System" by Doc Love, AKA Thomas Hodges
Although much of the advice shared in the book may seem antiquated now (e.g. getting a girl's HOME phone number), at the time I read it in 2002, it made for a nice introduction to red pill philosophy. Doc Love's theory on "interest level" still holds up. Essentially, it doesn't matter how interested you are in a girl, but how interested she is in you. If you want to attract female attention, you're much more likely to get it by being a "challenge" than by making grand overtures.
"The Mystery Method" by Erik Von Markovik, AKA "Mystery"
Without a doubt, this is one of the best pick-up guides ever written. I first learned of Mystery from reading another book, "The Game" by Neil Strauss. I didn't hate Strauss' book, but I found it to be a bit disappointing at the time, as it didn't contain nearly as much "how-to" practical knowledge as I'd hoped. Fortunately, Mystery ended up writing his own book, which more than made up for anything left out in "The Game".
"Bang" and other articles written by Roosh V.
Before reading Roosh's work, I may have known what it took to be a player, but through reading his work, I discovered what it meant to truly have game. For a long time, I made the mistake of making women my whole life instead of just a part of it. But Roosh's articles stressed the importance of becoming well-rounded by educating your mind, strengthening your body and developing seduction skills -- to not just be a player, but to have real game. Through him, I also discovered the Chateau Heartiste blog, another awesome resource for red pill wisdom.
Other books I've greatly benefitted from:
"The Prince" by Niccolo Machiavelli
Any book by Robert Greene
"The Art of the Deal" by Donald J. Trump
"The Manipulated Man" by Esther Vilar
I'm currently working on reading several other books, which I've really enjoyed reading so far and will surely add to a list of future recommendations once I've finished them.