Sunshine is the baker at her family coffeehouse and an ordinary person in a world that is much like our own, apart the magic, and the fall-out from the Wars between the humans and the Others - vampires, demons, and the like. But a decision not to join her family for a night watching films, instead having some solitude at the lake nearby her home town has consequences when she is captured by vampires, and forced to accept that she is not as ordinary as she would like to be. For one thing, no normal person who help another vampire to escape...
People have compared the book to Buffy the Vampire Slayer, or the Anita Blake books. There are some similarities to BTVS, especially when Sunshine accepts that she is not an ordinary person, but I feel the comparison with Anita Blake is completely wrong - Sunshine doesn't want her role as a fighter against the "dark forces" of the Others, unlike Anita Blake.
In some ways, this is a version of the story of Beauty and the Beast, and Robin McKinley has created a truly believeable, real, reluctant heroine and a world that seems very real.
There are quite a few things left unresolved at the end of the book - I really hope Robin McKinley writes a sequel!