My attempts at teaching English have been, ah, somewhat hit-and-miss over the past six months. The main excuses I have given (where directly blaming other people couldn't be got away with) has been a lack of training and of suitable teaching materials to hand. I've spent too many hours trawling the web through well-meant but practically useless free ESL sites, and probably-not-quite-as-many hours devising lesson plans of my own invention which 90% of the time have proven to utterly fail to engage students in any degree. When I've asked other ESL teachers for advice I've mostly been recommended to "unlock my inner teacher within" as it were - ie, do whatever I think best.
Although I was aware of the BBC World Service web pages devoted to teaching English, I had failed to notice the downloadable lesson plans, of which dozens are available on line and many seem to be of excellent quality. Likewise, I missed the link to the British Council's Teaching English site, which has even more lesson plans (aimed more at school-age children), and a number of very useful books in PDF all completely free to download. There are also a wide range of articles for the edification of teachers, and both sites make good use of downloadable audio to be used in conjunction with classes, if desired.
Both these sites are extremely well-designed, and I suspect that they contain material as good as if not better than the many pay-for-access ESL resource sites. And if you please you can still go back to one of the open forum "Hey, here's a totally awesome idea for class!!! It REALLY works!!" sites for a handy back-up when in need of a change of pace, ie: a totally awesome variant on Hangman or Eye Spy. (OK, a lot of the ideas posted on sites such as Dave's ESL Cafe are very good indeed, but you do have to read through an awful lot of half-brained stuff before you find anything worth embarassing yourself in front of a room full of bored students for.)
The one actual, physical text-book I have, which I got my mother to order me from Amazon, is "Rediscover Grammar" by David Crystal. Concise and conveniently pocket-sized, it says on the back that it "remains the ideal guide and reference for teachers and students" - and so it is. I don't teach from it, I just occassionally use it to avoid embarassment by discreetly dipping in to check exactly what a preposition is.