Dragon’s Blood by Ann Gimpel

After discovering she’s half dragon, Rowan figures it can’t be any worse than being related to the Celts. That’s the thing about assumptions, though. They come round and bite you in the ass.
The second book in a magic-laced, fast-paced, fantasy trilogy. With dragons.
I’d rather fight than study, but I’m stuck poring over dusty scrolls. I promised I’d learn about the dragon part of my magic, but I’m having a hell of a hard time believing there’s some concealed strain of power just waiting for me to kindle it. Meanwhile, my friends the witches are playing fast and loose with remaining hidden.
My Celtic kin won’t bother them anymore—at least I don’t think they will. But far worse things rove Earth than the Celtic gods. The Breaking has developed an energy all its own. The longer it runs wild, the harder it will be to contain.
Soon, very soon, no magic in the Nine Worlds will be enough to counteract it. Once that happens, the few remaining mortals will go first, but the rest of us won’t be far behind them.