Lady in Gil, Scion's Lady, and Lady Pain by Rebecca Bradley.
I'm not sure how these books ended up in my collection. They have used book store price markings inside the covers, so they came used. Possibly as a gift, I don't know.
The first two books follow Tigrallef, Scion of Oballef, who is supposed to go back to the island his people were driven out of by the Sherank, a vulgar people who mainly enjoy killing other people and being slovenly. Tig is a librarian and scholar, not the Trained Hero (tm) who has the Heroic Image, etc. But his older brother (the Hero) was injured, so he can't make it.
Tig gets back to Gil and gets taken under the wing of a rebel movement, who help him break into Gilgard (the castle) and steal the Lady (the MacGuffin). The Lady is a powerful magical object that is said to have built the nation of Gil from nothing, and the Sherank want it so they can continue to dominate the rest of the world.
One of the rebels is a young woman named Calla, with whom Tig falls in love.
The third book follows Tig and his family as they try to figure out the secret of where the Lady came from and how to send her back to the void. The Lady has taken residence inside Tig's head, see, and he wants her gone. She's quite vicious and shows no qualms about destroying an entire nation of people.
According to her bio, Bradley is an archaeologist. When Tig's family searches the ruins of ancient civilizations for hints of where the Lady came from, it's done with a careful attention to detail, reflected from her experience.
I'm of two minds on these books. They're well-written, and the story compels you to turn page after page. I loved the subversion of the usual Hero tropes and how Tig really just wanted to hole up in his archive and read his precious books. At the same time, Tig was a dumbass and I wanted to kick him quite frequently. I didn't like him much as a person, while as a character he was well-drawn. The third book, narrated by a character who wasn't Tig, was my favorite.
There were a lot of things I enjoyed about these books, and if you can get over an annoying first-person lead character, you might enjoy them, too. Or you might not find Tig particularly annoying.