The book that invented the genre and named it: historia, Greek for investigation. Heródoto sets out to explain the wars between Greeks and Persians and cannot resist a detour, so you also get Egypt, Babylon, Scythian customs and the occasional giant gold-digging ant. He tells you what he saw, what he was told, and which parts he does not believe himself, which for the 5th century BC is a shockingly modern move. Ryszard Kapuściński carried it as his only book on his first assignment abroad and never quite put it down (Travels with Herodotus).