Any review of sufficient length and thoroughness is likely to include at least some criticism, if for no other reason than the reviewer not wanting to look like a cheerleader. The NYT often selects reviewers with relevant domain knowledge, or who are themselves authors in a related genre. So opinions will be expressed, hobbyhorses ridden, and nits picked, even in glowing reviews.
You should be careful to avoid setting up a business rule that could overemphasize a few negative raisins in a positive pudding.
The idea of a binary positive vs negative is really troubling to me as far as an algorithm goes. It should very much be on that sliding scale that Metacritic or Rotten Tomatoes uses--sentiment analysis shouldn't return just a 1 or a 0.
You should be careful to avoid setting up a business rule that could overemphasize a few negative raisins in a positive pudding.