For most part this is the book that kicks things off. Grave peril is the book where things are set in motion, the stage is prepared and the actors reach for their costumes. 
 The story begins when Harry and Michael a christian knight (magical/ holy sword and all) go up against a slew of unusual powerful ghosts. Things go, as well as they ever go for Harry, till he is invited to represent the White Council (the "good wizards") on a vampires masqued ball. Combine that with old grudges, malicious plots and the introduction of Harry's Fairy Godmother and this go from bad to worse real quick. 
 In all this was a good book with the introduction of multiple new and cool characters and an the ever expanding world of the Dresden Files.