If you were to choose a book that you really treasure for the content, what would it be and why?

I think the why is the part that should be emphasized more. And I am focusing on content because sometimes certain books are treasured because they were gifts at some special moment by some particular person. I am not at all talking about that. I am talking solely about content that have impacted you profoundly.

It can be of any genre (also academic). Shoot!