Previous book here.
Harry hasn't been doing so well for the last six months or so...and adding to the fun, the White Council's senior members are voting on whether or not to kill him for what he did in the last book. The only thing saving his butt is a job offer...from Queen Mab, wanting him to find out who killed the Summer Knight, a mortal man working for the Summer Court. Why's she hiring someone for this job, you ask? Well, two emissaries were chosen, one for each side...and the other side's emissary is Elaine, Harry's first girlfriend who he thought went evil and died. (Not so much.) Their job is to investigate both sides's faerie royalty and try to prevent a war that will start if the power balance isn't restored...
While I don't feel like the investigative side of this is handled in the best way ever (essentially Harry finds the various queens and asks if they did it, and then just kinda comes up with the whodunit), it does end in a pretty explosive manner, and it's certainly gripping at the end.