The synoptic gospels and NT, but to choose one book from the NT, ...
The Epistle of James - as it has some strong counterpoints to other parts of the NT particularly the contrast of Pauline notions of grace with charity/works.
Emperor's New Mind, Penrose - probably my biggest take away from it was to consider how human perception works alongside, and co-mingles with, physics.
Web of Life, Capra - considering holistic nature of life, one's connection to the World at large; emergent patterns, complex structure birthing from simplicity (goes well with what I recall of Gleick's Chaos).
Zen and the Art ... - what is value, what is valuable to me, whither/whence/wherefore value; how should I relate to my children (long before i had any) and how to challenge them philosophically.
Republic, Plato - the first book that really set me thinking about the structure of society, about inequalities. And of course about stepping out of The Cave.
Koran & Hadith (partial readings, ie whole sections; couple of major hadith only) - [redacted]; but greater understanding of Islam and of religion in general; taught me to watchfully avoid being tainted by the labels people give themselves and look instead to their actions.
Mein Kampf (partial reading) - ideas have intrinsic moral value, that we should judge ideas on their merits and not by who has them; we should be careful about tarring people by simple association.
Art of War - preparedness, looking to supply lines, avoiding conflicts, not entering "battles" you know you'll lose (which I'd take as 'learn nothing from').
Christianarchy - what it means to be a Christian, who is and isn't "in".
Agrarian Justice, Paine - what is my place politically and economically in the world, whence do I derive the 'rights' to own what I do, whether such ownership is good.
Worth noting here that the ideas and impressions I got are unlikely to be what you will get, we react to books based on who we are, our frame of mind, moods, etc.. Most of these books I read as a late teenager, that at least in part boosts their impact. The impact is not necessarily the purpose of the book, quite the opposite in some cases.