If you are new to teaching, you should get your hands on an easy-to-read book that gives you the basics of teaching ESL A Course in Language Teaching by Penny Ur is a perfect choice. Ur gives you lots to think about as you prepare for your first class. Learning Teaching by Jim Scrivener is equally superb. You should also look at David Riddell's Teach Yourself Teaching English as a Foreign/Second Language.
Another good choice is How to Teach English by Jeremy Harmer. If you want some ideas on preparing lessons, I think Preparing Lessons and Courses by Tessa Woodward, is a terrific book. Highly recommended!
Becoming a professional ESL teacher means having a good understanding of language teaching methodology. The following books can help you learn the difference between the natural approach, the communicative approach and the cognitive approach. Earl Stevick's fairly recent Working with Teaching Methods is a beautifully written, concise summary of teaching methodology. Highly recommended. Meanwhile, Techniques and Principles in Language Teaching by Diane Larsen-Freeman gives you a clear picture of how several methods and approaches work in practice. Approaches and Methods in Language Teaching by Jack C. Richards and Theodore S. Rodgers is another great title.
A top-notch teacher should have several resource books of games and activities, to keep his or her classes interesting, and introduce some variety into every lesson. The best book on the market today is Five-Minute Activities by Penny Ur and Andrew Wright. This one is packed with all kinds of fun and interesting activities that can spice up your classes. A close runner-up is the recently published Humanising Your Coursebook by Mario Rinvolucri. It could almost be called Mario Rinvolucri's Greatest Hits, as it features some of Rinvolucri's best teaching ideas, along with a lot of new twists. Another recent book worth mentioning is Teaching Large Multilevel Classes by Natalie Hess, which has some very interesting activities for larger classes. I'm also very fond of Learner-Based Teaching by Colin Campbell and Hanna Kryszewska. This book has some unique, original teaching ideas I have yet to see anywhere else. Finally, The Recipe Book by Seth Lindstromberg is definitely worth a mention, a real treasure trove of great teaching activities.
What? you say Handouts us teachers can copy? Really? Yep. It's true. They're now making books of teaching materials and letting us do all the copying we want. Here are my all-time favorites: Discussions A-Z Intermediate, along with Discussions A-Z Advanced both by Adrian Wallwork, have an amazing variety of materials for discussing serious and philosophical topics with your students. Don't forget to order the cassettes for both the Intermediate and the Advanced. Wallwork has also created a super photocopiable book for teaching about special days of the year called The Book of Days by Adrian Wallwork, which also has its own cassette. If you want to wake up your class a little, try some of the photocopiable materials in Richard MacAndrew and Ron Martinez's Taboos and Issues. It's Magazine also produces some unbeatable packs of teaching materials.