As I've read through this thread, there are some things that caught my attention.
One was that certain books and authors are surprisingly (or perhaps expectedly) common in INTP's reading
The Tao Te Ching
Sun Tzu's Art of War
The Lorax
The Prince
A Wrinkle in Time
Ender’s Game
Brave New World
Stranger in a Strange Land
and various books by
Orwell
Ayn Rand
Dostoyevsky
Carl Sagan
Nietzsche
CS Lewis
Herman Hesse
I was also amazed at how many of the books people mentioned (beyond the ones listed) I had read as well.
My own list, with commentary.
Fiction
The Fountainhead + Atlas Shrugged by Ayn Rand- They profoundly influenced me, but I'm not sure in the long run if it was for the better. A better writer than a philosopher; a better thinker than a human being.
The Great Gatsby- F. Scott Fitzgerald. Like the wonders of the ancient world, human beings can build magnificent things in dedication to the imaginary. The first book that made me realize prose can be poetry.
Dune by Frank Herbert- More ideas than you can shake a dozen sticks at. Began me on the path toward exploring human potential.
Pride and Prejudice + Emma by Jane Austen. Austen was a brilliant psychologist long, long before Freud and Jung. Also, the most important decision most people will ever make is in whom they will marry. From a practical standpoint, this makes her works of more value than most "serious" books.
The Brothers Karamazov- F. Dostoyevsky. Jane Austen in a really dark mood.
Steppenwolf by Hermann Hesse. If you focus on the pain and despair, you might miss the transcendence and healing.
Illusions by Richard Bach. The only metaphysics that makes any sense.
The Importance of Being Earnest- Oscar Wilde. Not technically a book, I know. Possibly the funniest thing written in the English language, but I haven't gotten around to reading everything else yet.
Honorable mention to the collected works of Arthur Conan Doyle, Ray Bradbury, Tom Robbins, Michael Crichton, Joseph Heller, G.K. Chesterton and Alan Moore.
Non-Fiction
The Tao Te Ching by Lao Tzu. When nothing needs to be said, nothing is left unsaid.
The 48 Laws of Power + The 33 Strategies of War + The Art of Seduction by Robert Greene. The essential 3 works of Greene, who is Machiavelli, Sun Tzu, and Giacomo Casanova all rolled into one and updated for the 21st century.
The Law by Frederic Bastiat. When I first realized there can be a huge gulf between what is law and what is right.
Cows, Pigs, Wars, and Witches: The Mysteries of Culture by Marvin Harris. The logic behind all those seemingly inane customs and rituals.
The Ethics of Liberty by Murray Rothbard. Ah ha. So natural law is not "nonsense on stilts" after all.
The Transformation of War and The Rise and Decline of the State by Martin Van Creveld. The future of war, conflict, and the nation state, according to one of the most prominent military historians and philosophers. Only read if you want your world view shaken, not stirred.
Democracy: The God That Failed by Hans Herman Hoppe. When you want to skip having your world view shaken and go straight for the puree'. Yet it is so obvious you can't believe you didn't see it already.
Honorable mention to the collected works of CG Jung.