Oxford Practice Grammar

I refer to this book all the time when preparing English lessons for my high-level students, and my coworkers are constantly borrowing it, too. It explains hundreds of grammatical rules, including ones you use all the time but never learned about in school. (Why is it "big fat liar," not "fat big liar"? What's the rule for that? Better check the OPG.) Also includes a concise appendix on the grammatical differences between British and American English. Highly recommended.