I read this book 2 months ago, so I am foggy on the details. I also read books 1 & 2 of this series back in 2010, so I'm fuzzy on the back story as well.
Harry Dresden is a wizard-for-hire in modern-day Chicago. He frequently works with the police department on their "Special Investigations" unit. He has helped them stop rogue wizards, werewolves, and vampires. He's also kind of a smart-ass so he has made many enemies not only in the supernatural world, but on the police force.
Someone or something has pissed off the ghosts in Chicago. They are raging and rampaging all over town. The novel starts with Harry and Michael Carpenter, a holy knight, trying to stop an evil ghost in the infant ward of the hospital. They soon discover that the ghost has been tormented and driven mad by a torture spell. Then a bunch of stuff happens that I don't remember and don't want to spoil if you haven't read the book.
I really enjoy this series. It is like a hardcore detective novel mixed with the supernatural. It is reminiscent of the early Anita Blake: Vampire Hunter novels. The good ones. Before they turned into supernatural orgies. Not that Harry is a priest. Just not the total slut-bag that Anita turned into. I also love Harry's snappy one-liners and witty asides. I recommend it, but read Storm Front and Fool Moon first.