Bible and Koran for cultural influences all around the world
and if you want a good grip on the Bible get as old af a version as you can plus the newer translations both and the The Apocrypha
Constitution, and really the federalist papers as well for more insight into the mind set of many of the country's founders
OK this one if US-centric, but shift documents appropriately for whatever country you live in
I good Highschool/college 101 level textbook on physics, biology, and chemistry
Most people don't need to retain it all unless they go into science or engineering of some sort, but really things everyone should have a basic handle on
Illiad, Oddessy, Lord of the Rings, Shakespeare, Chaucer
just the joy of reading them and expanding one's mind, plus the cultural references that stem from them
The Hayne's manual on whatever vehicle you own, because the ability to do basic maintenance and repairs on one's vehicle is a very practical skill to have
Any decent cookbook for similar reasons, as well as a basic intro to sewing book (by hand or machine)
Plato's Republic, so much of the political structure in the Western world is based on ideas contained in this book
Fahrenheit 451, Starship Troopers (just pretend the movie never existed), 1984 all for insight on where society in different ways
Rules for Radicals, and the Communist Manifesto because regardless of which side of the political isle you fall on, ideas from these two get used in politics a lot
The Art of War, because even if military history and strategy aren't your thing, some of the ideas can be used in almost every aspect of life