My picks, some of which have already been mentioned. I would classify these all as "viewquake" books for someone who hasn't encountered the concepts in them before.

1. Godel, Escher, Bach - gets a huge credit for sending me down the rabbit hole of "what your brain is actually doing", though like others I'm not sure if I would like it as much on a second reading.

2. Meditations by Marcus Aurelius - Stoicism at its best, I count this as the most motivational book I've ever read.

3. The Sciences of the Artificial by Herbert Simon - retreads topics that are probably already somewhat familiar to LW readers, but still has one of the highest insights/page ratios I've ever seen.

4. 48 Laws of Power by Robert Greene - instrumental rationality in the social arena.

And one dis-recommendation:

1. A Pattern Language by Christopher Alexander - the concept of "design patterns" which gets quite a bit of mention nowadays got its start here, but this book is a mess. The support Alexander uses to back up his choice of patterns is laughably sparse, often completely wrong and picked to support his somewhat warped sense of eco-morality. Avoid.