I bought this book for when I was doing the Delta course and found it invaluable. There are entries for any and all English as a second language, or English as a foreign language related things. Some random examples are: connected speech, deductive learning, dogme ELT, pairwork, and reflective teaching. I like this book so much because the entries are short, concise and just give you the basic details you need to know without all the fluff. It's the perfect entry into a topic before you start digging further because it can give you a framework from which to work.
Plus, it's HIGHLY quotable for any kind of research paper that you're doing. I can't recommend it enough for any English language teacher.