This is a series.
A world shared by three native humanoid races - the cavern-dwelling Gdemiar, elvish Fiia, and warrior clan, Liuar - is suddenly invaded and conquered by a fleet of ships from the stars. Earth scientist Rocannon is on that world, and he sees his friends murdered and his spaceship destroyed. Marooned among alien peoples, he leads the battle to free this new world - and finds that legends grow around him even as he fights.
Published in 1966, it appeared as an Ace Double novel alongside Avram Davidson's The Kar-Chee Reign. The novel is one of several of Le Guin's works that take place in the same universe and have relationships among characters and history, works that readers have termed the Hainish Cycle.