Tehanu

Tehanu: The Last Book of Earthsea

In Tehanu, Ursula K. Le Guin returned to Earthsea, writing a story she described as the missing fourth leg of a stool. Tehanu is the story of Tenar and Therru.

Fourth book in the Earthsea series by Ursula K. Le Guin.

Not actually "the" last book in the series in the series, despite the title.