The bible of value investing. Graham's framework for thinking about stocks as fractional ownership of businesses.
Mr. Market is the key concept. The market is like a business partner who shows up daily offering to buy your shares or sell you his, and his prices swing wildly based on his mood. Your job is to take advantage when his prices are irrational.
The margin of safety principle applies beyond investing. Always leave room for being wrong. Don't rely on things going perfectly. Conservative assumptions compound.