What aspects of linguistics are necessary or good for natural language processing? What aspects of linguistics are necessary or good to know for natural language processing? What references do you recommend for studying those aspects? Thanks!
NLP is a big place, you might want to be more specific.

Within information retrieval, stemming is a linguistic idea that has become useful as a heuristic means of reducing vocabulary size.  As a practitioner I learned about it from An Introduction to Information Retrieval.