The Last Planet by Andre Norton

In the waning days of a vast interstellar empire, a lone ship of the Patrol crash lands on a minor, very out of the way planet. Quickly scouting around their crash site, they find evidence of a long vanished high tech civilization in the Sealed Cities, along with nomadic hunter-gatherer level groups of humans. Looking for better shelter to tend their injured personnel, the rangers enter one of the cities, only to find it occupied by another group of refugees and ruled by the Acturian Cummi, a master telepath, one who is not above overpowering and directly controlling other peoples minds, who is bent on becoming the sole ruler of the planet. Zinga, a member of the ancient historian race of Zacathans (a reptilian race that populates many of Norton’s science fiction works), and the human Kartr, both high order telepaths themselves, though not of the strength of Cummi, end up in a memorable mental battle with Cummi.