Programming: C Traps And Pitfalls. Most programming books are focused on the happy path; in reality we spend most of our time in the unhappy one, trying to fix or debug something. That book made it a lot more hospitable to be in that state.

General non-fiction: Extraordinary Popular Delusions and the Madness of Crowds; http://www.gutenberg.org/ebooks/24518

It's surprisingly relevant today and many of the anecdotes are fun. Helps you have a healthy skepticism but also acceptance of fads.

Fiction: Pratchett. Hilarious and deeply humanist at the same time.

Fiction and politics: Ian M Banks; fully automatic luxury space anarcho-liberalism. With lasers.