Books for English Teachers
I use Jeremy Harmer's blue book as the textbook for my introductory TESL course. It's non-technical and an easy read. Perfect for someone who wants to start teaching.
The Practice of English Teaching and Contemporary Linguistics are much deeper. They're ideal for an experienced teacher who wants to add a solid foundation to their teaching.
How to teach English by Jeremy Harmer (Blue Book)
The Practice of English Teaching by Jeremy Harmer (Red Book)