Harry Dresden is a detective, a former cop, a Wizard of the White Council, and Winter Knight to Mab, The Queen of Air and Darkness.
And this is his fifteenth adventure, leaving the reader coming in cold working to figure out a great deal of backstory.
Harry's immediate problem is that he has a magical parasite in his head that is causing terrible headaches and will eventually kill him. There's a limited number of people who can help him, and they're not responding to his messages. Then Mab shows up at his Demonreach lair, with such a deal. She's got a temporary fix for Harry's problem, to allow him to function away from Demonreach so that he can do a little job for her--or rather, for Nicodemus, whom she owes a favor to. If he completes the job successfully and returns alive, then she'll deal permanently with his parasite problem.
Harry hates Nicodemus, but it's not just his life at stake; the parasite will go after everyone he cares about after killing him. He's not happy, well, even less happy, initially, when he finds out he'll be working with a warlock (unlicensed wizard, or something like that), Hannah Ascher, as well as another, even darker, character called Binder. Oh, and his first assignment is to recruit an old acquaintance, Anna Valmont, Anna's a thief, and her skills are going to be needed for this job, which as it happens is burgling the Greek god Hades' private treasure vault.
The writing here is nothing really exceptional, but it's perfectly competent and smooth. The problem is that because this is a Hugo Best Novel nominee, I'm coming into the series at book number fifteen. At this point, the book relies on the fact that everyone reading it knows the major recurring characters and the world they live in--and I don't. And sadly, without the backstory, I don't care. There's nothing here so compelling that I really wanted to keep reading. Regular fans of the series, I've noticed in online reviews, mostly feel very differently, but in a very real sense, we're not reading the same book.
If you haven't, or don't want to, read the previous fourteen books, skip this one.