Ancient Promises by R. E. Butler

When dragon shifter Eivross balks at his king brother’s desire to wage war against another nest, he chooses to be exiled instead of helping his brother take innocent lives. Exile for a dragon shifter means finding a new place to call home, and he chooses Cider Falls.

Diem loves Cider Falls and working in the bookstore she owns, and she’s even happier now that she’s picked up a volunteer job at the school library. When a stranger comes to town, Diem knows he’s her truemate, and not just because he shifts when they meet and rips the front door off its hinges.
But Eivross won’t take Diem as his mate because his brother is dangerous and he’s worried she’ll be caught in the crossfire. When a spurned female dragon tells his brother where Eivross is, he shows up with an army, intending to take Diem for his own mate and kill Eivross in the process. Can Eivross keep Diem safe against a horde of angry dragons, or will he lose the most important treasure of all?