Gedo Senki. by Goro Miyazaki (2006). Studio Ghibli Issue English Synopsis.

This appears to be all from the third book, "The Farthest Shore", from what I remember.

Yeah, from the 3rd book. Though terru or whatever doesn't feature in it and they've rejigged a little. I thought it was going to be the first book, but the third is better anyway.

I'm nearing the end of the 3rd book now. Tenar hasn't been in it so far. She's from the second book and I really liked that story. It was told from her perspective rather than Ged's. I'm not sure who Therru is. Now I really want to finish the book.

Ah, I remember now - Therru is actually from the following books (the real name of Tehanu, I think). Looks like it's a whole mixture of the stories.

The movie is a mix it seems of the fourth book, Tehanu, and the third, The Farthest Shore. Tenar and Therru do not appear at all in The Farthest Shore, while all the characters appear in Tehanu (Arren or Lebannen appears briefly in Tehanu).

I think it's meant to be based mainly on The Farthest Shore, but it takes elements from the other books, such as characters from Tehanu and I think some stuff from A Wizard of Earthsea too. Clever idea really.
I hear it isn't the same as the books at all, but I'm sure that it's still a great film, if perhaps more enjoyable for those yet to read the books.