White Night. The Dresden Files, Book Nine

Meet Harry Dresden, Chicago's first (and only) Wizard P. I. Turns out the 'everyday' world is full of strange and magical things - and most of them don't play well with humans. That's where Harry comes in. A series of apparent suicides rings alarm bells with the police, and Harry is hired. At the first crime scene he hits pay dirt, discovering an unmistakable magical taint. There's also a message especially for him, and it ain't pretty.
The 'killings' will continue if Harry can't halt his tormentor, but the evidence implicates his half-brother, which just doesn't add up. Unfortunately Harry's digging around attracts some powerful vampires with a stake in the result. Soon, whichever way he turns, Harry will find himself outnumbered, outclassed and dangerously susceptible to temptation. And if he screws up, his friends will die. Magic - it can get a guy killed.