At a minimum, it should give the broad outline of the book. My proofreader is not a mystery reader and doesn't read for content but for errors. However, one review in the September issue by Eden Embler made her sit up and take notice. That's the response I'm looking for. But Mrs. P moved to the top of the stack.
Publishers are, for the most part, thrilled to send out books for review.
They do tend to send the books they are pushing, and you have to dig for the new mystery author who is somewhere in their mid-list. This past May, Book Expo was in LA, and I made a lot of contacts with small-press publishers of mysteries. I'm very interested in giving exposure to new authors and new publishers. The big guys have publicity budgets but the smaller publishers and the mid-list authors tend to get lost in the shuffle. If you see any mystery authors out on tour, unless it's Patricia Cornwell or Mary Higgins Clark, the chances are they are footing the bill themselves.