A solid 4 Stars
This one was harder for me to rate. I can see how many people will say this is better than the previous books in the series. It is certainly better than the second and probably better than the first. Overall, the story is more involved than the first two and the action and suspense level has been ratcheted up a few notches. For me though, it still lags behind the first for the simple fact it drifted from an Urban Fantasy into a full out Fantasy.
In Grave Peril, the feeling of Harry Dresden, a PI who happened to be a Wizard was lost. The PI/Detective/Crime mystery portion of the previous books was absent as were the majority of the "mortal" friends.
Regardless, the story and writing was superb and it gets a hearty 4 Stars.