"Grave Peril" is a really strong entry into the series, with interesting new characters (even though way too many of the characters by now have names beginning with M - "Malone" and "Marcone" especially are too close) and even some female characters who aren't - gasp - stunningly beautiful all around.
That said, Harry's so-called chivalry seems to be getting even worse. He's basically incapable of seeing females as adult people who can take their own decisions and be responsible for their own mistakes. At least some characters call him out on that.
What the mysterious happenings are related to isn't really so hard to figure out (it's not as if Harry would be doing cool stuff in between the books if it weren't somehow plot-relevant), but there's still enough twists to keep readers happy. And the latter part of the book, concerning the vampire's masquerade ball, is just wow - it's staggering just how far Butcher is willing to go with his characters and how much he's willing to change the direction of the series so far. That alone is worth five stars.
Though I'm seriously tempted to deduct one for Susan. I was pleasantly surprised by her competence and reason in the second book, but then, all she had to do there was drive a van. In "Grave Peril" she does something so monumentally stupid and ill-advised (no, I'm not talking about the memory thing) that you seriously cannot say the consequences are too dire - if anything it's amazing how long Butcher manages to delay the consequences. And of course Harry feels responsible even though for once he isn't, and we have do endure his whining about it at length, in this book and the next ones.
Other than that, some of the usual mistakes - at one point the character "Rachel" from "Storm Front" turns into "Paula" and then back into "Rachel"; at another, the vampire Thomas Raith meets Harry's eyes "for a moment - almost long enough to let me see inside him", even though we've heard five minutes ago that vampires don't have souls and therefore can't soulgaze. Also, Harry didn't know anything about his mother in "Full Moon" but now knows - and apparently has known since childhood - that his mother promised him to a fairy when he was little.
Still, as I've said, the vampire's ball (and Thomas Raith) are worth five stars.