[Eric18]

Brown's book remains the gold standard, but it can be a bit dry.

The most practical - and engaging - book on theory that I've read comes from Cambridge University Press called Global Englishes. By examining how English expanded across the globe, investigating how English is used in different countries and contexts, and comparing the types of common "mistakes" while learning English, the author creates a fascinating picture of changing world. I learned some history, reviewed some linguistics, and developed a few new techniques. It also helped me better tailor my lessons to the individual needs of students based on their goals and aspirations.