Rocannon's World

A spoiler or two in the paragraph below:

This is the story of Rocannon, an ethnologist sets out to find the base on Fomalhaut of an enemy to the League of All Worlds, the planet of Faraday. His interactions with the various lifeforms on the planet change him and he is eventually rewarded with the gift of telepathy. Rocannon saves the planet, but not before experiencing the pain of all the beings who were killed in the process through the gift he'd been given.