This is an intermediate to upper intermediate level book for students of English, NOT native speakers. It is extremely good at its job and I have even used it successfully with advanced level students as it uses language and grammar which is totally relevant to the genre of business English for non-natives.
I thoroughly recommend it for any teachers of ESL, although after going through the first, basic chapters, I normally let students choose which of the others are most relevant to them as they can be quite repetitive (not always a bad thing). The exercises are fairly standard (!) but used with a little imagination they can be spiced up and tailored to your individual teaching style. Answers in the back make it possible to be used effectively as a self-study book and there is also excellent online back-up material providing useful practice/homework tasks.