Relearn the essential rules of the English language, from grammar and punctuation to sentence construction, parts of speech and how to improve your spelling and vocabulary.
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.
English is my second language and some chapters were a nice addition to the process of mastering the language - they cleared out some doubts about how to say something correctly in English. Worth browsing through :)
The entire spelling section is wrong, quite frankly, and based on old myths perpetuated by people who don't understand the history of the English language and how that impacts it today. Other than that, an interesting read and the "pitfalls and confusions" section is potentially a useful reference (for example when you forget the difference between affect/effect or complement/compliment).
This was a bit of a slog, because it is a technical read. However, it is methodical in the way it maps out the rules of English. It teaches you how to speak, write and communicate properly. I enjoyed it and will be using it for reference.