Ishmael by Daniel Quinn
It is hard to explain Ishmael. It has a man who feels something is missing in his life and it has a monkey (gorilla actually). What more do you need? I am well aware that this sounds horrible to most people. I was skeptical at first too when assigned to read it for a Criminology class. It is about environmentalism, it is about human nature, it is about knowing yourself, and living in harmony with nature. Still sound awful? Yeah probably. It's hard to explain but even through all these horrible new-age precepts, it is in fact an amazing book. I wouldn't call it "life changing" although it definitely made me think more about my life. It describes all the bad things that man has done to the world, environment, etc...but does that doesn't mean he is a criminal, it was his evolution, his growing as race. Of course, that doesn't mean that we need to stay on the same track, we are evolved enough to realize we need to change without demonizing ourselves.
I am still explaining it badly. I would suggest you pick it up and read it sometime (it is short), I don't think you will be disappointed. I plan on re-reading it soon.
BTW - there are other books in the Ishmael series: My Ishmael and The Story of B. These aren't nearly as good and haven't actually partially ruined the magic of the first book for me. On their own they might have been OK, but as sequels to Ishmael they were disappointing.