Excellent idea, in principle.

At risk of violating the KISS principle, I would suggest that in this case it might me useful to add another dimension to the vote.

For a particular book, as opposed to a review of said book, allow voting along certain categories such as tutorial, reference, advanced, culture, etc. These votes need not be mutually exclusive, so that a particular book might embody or lack more than one quality.

For books, anyway, such a cross section of voting would certainly be more informative to a prospective reader.

Matt