I'm at a loss! If I'm talking about several different types/kinds of something, should the 'something' be pluralised or not? For instance, an Ngram search shows that different types of conflict is far more common than different types of conflicts, but on the other hand, an Ngram search for different types of book(s) gives the opposite result! I have also searched for the same string with war(s), fight(s), concept(s), and flower(s), with the same varied – and seemingly random – result.
So, what are the rules (if any!?) here?