Two books - The Internet and LanguageTeaching - ESL
It's one of the seminal books for all TESOL courses and teachers. See his blog here.
Here are a few extra reading/resources. What great teachers do differently? epub version. Also Teacher Made Assessments.
I used this book when teaching pre-service teachers in Ontario, Canada. Key Questions For Educators.
Short, non-jargon filled chapters about essential questions anyone serious about education as a career should think about.
Highly recommend the small but powerful book. Here's a chapter excerpt - Question 24. What is student engagement?