Intro micro-economics and macro-economics text book - the one best single source of all key economic concepts: opportunity cost, marginal utility, LRAC etc. These are the foundational element that every one should know, not just economists and entrepreneurs.
Do you have a suggestion for a specific book or books?
Lean startup by Eric Rise, made me go out and test the idea I had before we started building, made us pivot to something much better than what we were doing.