In my time as a TEFL teacher this book came in useful on numerous occassions. TEFL textbooks have notoriously shoddy guides to grammar, which are often detrimental rather than helpful for student and teacher. Swan doesn't have any practice exercises, but everything is laid out plain and simple, from the most straighforward grammar points to the complexities of cleft sentences.
As a teacher, especially in my first year, I would have sunk without a trace if it hadn't been for this book. You simply cannot do without it as a TEFL teacher, or as a serious English language student. There's not much more to say about it than that!