Hm… you mean, one that takes critic reviews into account and not just user opinions? I’m not sure whether a direct equivalent to Rotten Tomatoes exists if that is what you are looking for, but if you simply want a book review site, then I recommend Good Reads and Library Thing. You can find decent book reviews on Amazon as well.
I entered your question into Google and came up with this: You’re Welcome. Of course, it’s British (drawback?), but it includes American books, and film, and theatre.
@ZAGWRITER‘s link was meant to direct us to The Omnivore. Fairly interesting little website, though I haven’t the chance to give it a thorough look-over.