1. J.R.R. Tolkien: The Hobbit / The Lord of the Rings
2. Eliezer Yudkowsky: The Less Wrong Sequences
3. Michele Boldrin & David K. Levine: Against Intellectual Monopoly
4. Olivia Fox Cabane: The Charisma Myth
5. Marshall Rosenberg: Non-Violent Communication
6. Eugene Gendlin: Focusing; Connirae Andreas & Tamara Andreas: Core Transformation
Tolkien, because he got me really into fantasy.
The Sequences woke me from a certain super-postmodern thought, where I basically felt that I could believe in anything as long as I came up with a sufficiently clever argument for it. They made me realize that there are actual mathematical laws regarding the kind of evidence I must have witnessed in order to start believing in something, if I wish to have correct beliefs. Also convinced me about AI being the biggest thing in the history of humanity, and got me on the career path that I'm still on.
Against Intellectual Monopoly bolted me from a very strong, principled "all online piracy is wrong" mindset to one where I later ended up as one of the founding members of the Finnish Pirate Party. It was also my first introduction to pro-market thinking and theories, with me having grown up in a climate that was very left economically.
The Charisma Myth got me to realize that one can be charismatic without being extroverted, and that being charismatic doesn't necessarily mean saying interesting things all the time. It made me understand that just being present and paying genuine attention to the other person were things that could already give you considerable charisma, and furthermore these were some skills that I already possessed. It meant the start of my conversations with people going from the constant question of "oh now what do I say next?!?" to actually being present in the moment and not worrying so much.
Non-Violent Communication started me on the path where I can actually usefully work with the underlying needs and beliefs behind my emotional reactions, instead of treating them as atomic reactions that I can do very little about.
I'm bunching Focusing and Core Transformation together, as I think of them as two books that discuss variants of what's fundamentally the same technique. I've found the Core Transformation version of it really powerful during some of the last few months, on the order of taking maybe half an hour to permanently cure psychological issues that had plagued me for decades. That said, I suspect I wouldn't have been able to properly use the technique had I not first read Focusing and practiced with the instructions there.
Honorable mentions:
* David Friedman: The Machinery of Freedom. The book which further shook my very leftist thought, and got me to realize that libertarians also have some pretty damn compelling arguments, and that I'm not really qualified to say who's right. Decided that I'd avoid taking any strong positions on economics from now, given how complicated the whole thing is. (have had varying levels of success with this decision)
* Pema Chödrön: The Wisdom of No Escape: How to love yourself and your world. Only read this book in the beginning of this year, but it has been very powerful in changing my thought and putting me on a path of greater self-compassion and self-acceptance.
* John Yates & Matthew Immergut & Jeremy Graves: The Mind Illuminated: A Complete Meditation Guide Integrating Buddhist Wisdom and Brain Science for Greater Mindfulness. The best book on meditation that I've ever read. This is also something I started reading *very* recently (many thanks, Juha!), but I'm already provisionally ready to nominate it for an honorable mention, because it's meditation instructions have been super-useful. They've helped install an automatic habit of my mind automatically dropping any lines of thought that will only be harmful (e.g. worrying about things that I have no control over); time will show whether that habit will last.