I'd been warned that this wasn't King's best. But they're all some degree of fascinating, and the interaction between Mary and Sherlock is always stellar, and I have to read them all, right? Right.
I think the reason this one is not as great as the rest is for two reasons. First, because she's rehashing the old Hound story. It's dangerous territory, since she's already rehashing Holmes to begin with.
But second, and chiefly, she's set this in a locale that sounds simply... boring. If Mary isn't cold or wet, she's exhausted or irritated. The love the inhabitants of the moor have for it does not shine through. And it should! It may be a dangerous place, filled with slippery bogs, but its windswept majesty and haunting beauty should be extolled by the author, not denigrated at every turn.
In the end, there's just too much tramping around and not enough real action.