Book number five in the Dresden Files sees Dresden caught up in an incipient war between the fay courts of summer and winter.
This is the book in the series I enjoyed most so far. It doesn't have as much setup as most of them do, getting to the action pretty quickly.
The story also rocks along without pause and has some neat stuff about the (wizard's) High Council on Harry's case (due to the fact he pretty much started a war between the wizards and vampires in the last book). The politics and mythology of the fairy courts is really well built up, too.