Which book had the greatest influence on you?

This is something that's being discussed in various parts of the internet right now, due to something a famous US politician said recently.

Which book (fiction or non-fiction) has had the biggest influence on your worldview and how you live your life?

For me, it'd be Douglas Hofstadter's Gödel, Escher, Bach, with a close second being Chris Chabris' and Dan Simons' The Invisible Gorilla.

The Hitchhiker's Guide to the Galaxy trilogy (all 5 books) are absolutely the books with the greatest impact on my life ever.

I got the first book as a christmas gift when I was 13 (over 30 years ago). I started reading it that evening at my grand-parents' house. I only put it down to sleep that night. Then, I finished it the next day. And as soon as I was done, I started over.

I got Restaurant at the End of the Universe from a friend (borrowed, then he gave me the book) and read that in about 2 days. Then read Life, the Universe, and Everything shortly thereafter. I think I read all three of those books through at least a dozen times over the next couple months.

I got So Long and Thanks for All the Fish when it came out and read through that immediately. Of course, I then had to re-read the first 3 books and read the 4th again after that.

I somehow hadn't heard about Mostly Harmless until my friend showed up at my house with a hardback copy of it signed by Douglas Adams (and stamped with a 42).

Adams' sense of humour has shaped a lot of my own and his views on the world we live in and life in general have served as a very strong navigation point for me throughout my adult life.

Machiavelli's The Prince. His work on power, influence and human nature is as relevant today as it was 500 years ago.
Also What Is Your Dangerous Idea edited by John Brockman

TjD