The Longman Student Grammar of Spoken and Written English: Workbook (Grammar Practice)

An invaluable reference book if you want to get right down to the real nitty-gritty of grammatical usage. Detailed and extensive, descriptive rather than prescriptive, and corpus-based, this is a grammar for the C21st.

This book really is a gem given its size and the amount of grammar it covers. It is invaluable for answering all of the difficult questions TESOL teachers may face, e.g. phrasal verbs, modality and compound nouns etc. It also explains why written and spoken grammars are so different, and should be treated as so. All of the examples used are from a corpus of real language. I have used it both in my undergraduate and now post-graduate work. It is simply the best.