Two books that greatly influenced me were "
A Problem From Hell: America and the Age of Genocide" by Samantha Power and "
To Kill a Mockingbird" by Harper Lee.
I read Harper Lee's book when it was originally published in the 1960s and it made me look take another look at my childhood. Ultimately, it was the original stimulus for my becoming an active part of the Civil Rights Movement in the 1960s and made me look more closely at the world around me.
I also read Power's book when it was originally published about 2002. I always had associated genocide with the Holocaust of World War II, but Power made me realize that genocide didn't stop with the end of World War II. She also made me realize that American politicians were paying lip service to "never again" as they sat and watched it happen repeatedly. I became an advocate for U.S. intervention to prevent genocide. I advocated, for example, intervention in Darfur, which I considered a worthy role for America, even if we had to intercede alone.
Unfortunately, you asked for one book and I've given two. There are a number of other books that over the course of a lot of years of life that have greatly influenced my behavior. Alas, unlike crich70 who was positively influenced by the Bible, I found it to be a very negative influence. It is interesting how the same book, be it fiction or nonfiction, old or new, can positively affect some and negatively affect others. The human mind is wonderful in that regard.