Watchmen (1986)

HBO's Watchmen is not an adaptation at all but a continuation, set 34 years after the comic and built on the assumption that you know it. That makes Alan Moore and Dave Gibbons's 12 issues the essential companion: the show's twists, symbols and returning characters only carry their full weight once you have read what came before. The comic also stands entirely on its own, a dense, carefully structured story about who watches the people who claim to protect everyone. Read it first, then watch the series.