Holy Blood, Holy Grail - Michael Baigent, Richard Leigh, Henry Lincoln I must have picked this book up around the time it came out, which would have made me a sophomore in high school.

When I became a "real" historian, I came to realize just how much of it was hyperbolic tripe but even as a youth, I understood that 90% was speculation (to put it kindly).

The two things it did do for me were:

1. Reveal a Dark Age world of Europe that I hadn't encountered before.

2. Gave the alternate-history buff in me a whole new set of "what-ifs" to think about.