1 - Discussions That Work - Grammar Practice Activities and Teaching Listening Comprehension all by Penny Ur. The books written by Penny Ur are consistently good, and these three are all very useful; particularly the first two.
Discussions That Work is one of the most useful TESOL books for getting your students speaking. It has many ideas of tasks that will encourage the students to talk, and shows the teacher how to structure a classroom discussion. The first part of the book is teaching you how to teach conversation; the second half has many discussion activities. Most of which I've tried with success.
In Grammar Practice Activities, Penny Ur had the good idea of listing the English grammar points, including tenses, and for each point created a chapter of grammar practice activities. A good book to use when you are short of time and want ideas for teaching adjectives, relative clauses, the past simple tense, or any other grammar point.
Teaching Listening Activities will perhaps appeal to less people, but a careful study of this book will provide you with a great many ideas for teaching listening, and learning how to develop your own ESL listening activities.
2 - Practical Techniques [For Language Teaching]. This is one of the best TESOL books I've seen that introduces the teacher to the basics of how to teach ESL. Written by Michael Lewis & Jimmie Hill. It covers many principles of language teaching which can be adapted to many classroom situations, but always remaining practical in its approach.
3 - Teaching Tenses by Rosemary Aitken. Number three in my list of TESOL books describes the different English tenses, notes various problems learners face with each particular tense, and then lists ideas on how to teach each tense. Very useful, for understanding the differences between tenses, giving ideas on how to explain these differences, and with ideas on activities which are suitable for teaching different tenses.
4 - Teaching English Pronunciation by Joanne Kenworthy has practical ideas on teaching English pronunciation. Few TESOL books deal with this topic successfully, but this one does. I placed it at number three because teaching English pronunciation is an area which is often neglected, and this book shows how it can be successfully incorporated into the ESL class. The first half of the book looks at principles of teaching pronunciation, and offers many English teaching ideas. Not only for practicing the pronunciation of individual words, but also for practicing rhythm, stress and intonation. The second half of the book looks the particular problems that speakers of several languages have when learning English.
5 - Grammar Dictation by Ruth Wajnryb. This book is different from most recipe type TESOL books in that instead of offering many esl activities around a connecting theme, it focusses on a single idea, and then gives many examples of how to use it. Grammar dictation is also known as dictogloss, and it is a powerful technique for teaching - and learning - to use grammar accurately.
The basic procedure is for the teacher to read out a short text at natural speed - after first teaching any new vocabulary, and generating some interest in the topic. The text should be read twice while the students note down the main content words. Next, in small groups, the students reconstruct the text. The main point here is not copying the original, but in constructing grammatically accurate and textually cohesive texts. Then final stage is analysis and correction.
Of course, you could - and should - create, or find your own texts, but having the book does give you a collection of pre-prepared activities, focussed on particular grammar points and graded according to level. If you've never tried grammar dictation before, it's well worth a try.
6 - How to Teach English with Technology by Gavin Dudeney & Nicky Hockly. This is an interesting introduction to using technology in the classroom, and the book contains a CD-ROM with lots of extra material, including teachers talking about their personal experiences of using ICT in the class. The book contains ideas for using websites, webquests, class blogs, email, wikis and internet based project work.
7 - Discover English by Rod Bolitho & Brian Tomlinson. This is one of the few TESOL books I've seen which focuses on improving the teacher's level of language awareness through a series of questions. In the second half of the book extensive answers are given which explain the grammar of English. This book is only for those who wish to improve their own level of grammatical awareness, and there are no classroom activities. However, any improvement in a teacher's knowledge of the grammar of English will certainly improve their ability to teach.
8 - Teaching Writing Skills by Donn Byrne. The book is a little old now, but it's one of the best TESOL books I've seen on teaching writing. It is aimed at the general English class, both for teaching English to adults and teaching English to children. The book contains both practical principles and many ESL activities to use in the classroom. This book is not aimed at those taking more advanced exams.
9 - How To Teach English by Jeremy Harmer. This the recommended textbook on many TEFL certificate courses around the world. It is comprehensive and the new edition comes with a DVD. It it more complete than Practical Techniques - which I recommend above - although it lacks that book's readability. Nevertheless, it is a book that is extremely popular around the world, for good reasons.
10 - Practical English Usage by Michael Swan is the classic teacher's reference book. I hesitated to include this on my list, not because it isn't good - it is very good. But because so many teachers are already so familiar with this text. However, new teachers, and some more experienced teachers, may not be aware of its existence, therefore I include it as number ten on my list, but it could just as easily be number one.