Very strongly recommended to me by a colleague upon hearing I was going away to do my CELTA certification. First of all, it is as much a grammar book as a biography is a history book. What it is is a delightful roller coaster through all the grammar points any English teacher will need, written in an easily quotable language that is accessible to even the most unenthusiastic learner. Cover it in a protective plastic because it will fall apart, if not from personal use, then from a continuous lending and exchange of hands.