Ivan A. Sag

This is the standard textbook for learning Head-Driven Phrase Structure Grammar (HPSG). If you're interested in a formally rigorous (in the mathematical sense) syntactic framework that it is possible to implement on a computer, HPSG is a good place to start. The exposition proceeds incrementally, with successive chapters elaborating on a grammar of English. There are exercises at the end of each chapter which make Syntactic Theory suitable as a textbook for a graduate or upper-level show more undergraduate course.

(Full disclosure: one of the authors is my PhD adviser.) show less

It's the bible of linguistics for computational linguists.