Learning Teaching by Jim Scrivener
This book is so useful, covers everything from what to do on your first day to how to deal with teacher burnout to sample lessons to everything else. He also writes at One Stop English
if you want to get a feel for his style. If you're looking for someone to tell you exactly what to do, look somewhere else. His is more of a thoughtful, trial and error, "this is what I did, but your results may differ," kind of style. I am so glad I bought this book.
After that it depends on what you're interested in, the How to Teach... series is pretty good and if you're looking for a grammar book I think Raymond Murphy's Grammar in Use Intermediate has great examples for just about everything. Or check out Scott Thornbury's books or website for a completely different approach.