I'd say the typical recommendation is Viktor Frankl's "Man's Search for Meaning". I do recommend it, it's a great book. However, for me personally, the book that truly triggered my re-evaluation of life (my own and in general) was "The Selfish Gene" by Richard Dawkins. Lots of things instantly fell into place. It may not blow your mind the way it did for me, but read it anyway -- it's worth every second of your time.