The fourth in the Mercy Thompson series by Patricia Briggs.
There's quite a bit going on in this one. On a personal level, Mercy is still trying to deal with what happened to her at the end of Iron Kissed, as well as how that affects her relationship with Adam.
Stefan shows up, almost dead, both to warn Mercy and needing her help. His mistress is punishing him, ostensibly because of what he and Mercy did in Blood Bound. Of course, she also has other motives, but those don't become clear until much later. So when an old friend shows up asking for help with a ghost Mercy agrees in order to get out of town.
There is rather a bit more to it than a ghost! Which results in Mercy and the were having to deal with all kinds of vampire politics.