This is the best textbook for teaching e-mail English I have come across. Even allowing for the field being deplorably narrow, Paul Emmerson's "Email English" stands in its own right as a well-designed and useful book. I have used it with enthusiastic and less enthusiastic students of Intermediate level and have had consistently successful lessons with good feedback - even from students who felt they ought to learn more about writing natural sounding e-mails but didn't approach the topic with any eagerness.
The book starts with notes to the student and notes to the teacher and goes on to three introductory units. There are thirty two units altogether and these range in content from basics, like opening and closing e-mails and checking understanding, to more difficult topics like different levels of formality. There is a "phrase bank" which gives students a quick reference for language covered in the units - divided into sections and styles in accordance with the text - and an answer key, which is particularly useful for students (and in my case, the teacher) when working out, for example, the "Missing Words and Abbreviations" exercises. Thx was clear to everyone. Otoh, on the other hand, looked like a surname to all my Japanese students.
The general tips for students in the introduction are clearly set out and provide guidelines that, I'd guess, we'd all like to see used in e-mails by native speakers as well as English language learners. The style is clear and the advice is excellent.
Each unit is two pages long, covers a specific aspect of e-mail writing and can be used in isolation if you want to cover that particular point in a more general writing course. Each unit can be comfortably covered in a sixty minute lesson, allowing for individual questions or difficulties, or more quickly if you have a very able group. The units are generally split into an introductory section, which defines terms, sets up problems or situations, and the rest of the unit, which consists of exercises to provide students with the skills they need in the given situations - for example, gap fill, matching, substitution and rewriting. I think the exercises are the strength of this book. They actually do enable the student to practise the required skills.
One thing I feel Mr Emmerson does exceptionally well is to tackle the problem of correct tone. This is a difficult area to teach because cultural differences make students relatively insensitive to appropriate levels of friendliness, politeness and directness. If it's polite in your own language/culture, then it's not unreasonable to expect that it's polite in a business contact's culture but this is not always the case. In unit 1, "Formal or Informal", and units 28-30 he tackles formality, brevity, politeness and directness. There are clear examples to illustrate the points and questions, and exercises to get the students to think about the effect of, say, this brief and direct e-mail - Does it sound aggressive? Well, yes, the example given clearly does. Students then try reading an e-mail with and without the friendly connectors and decide what the difference is. They can then rewrite an e-mail and include some phrases, from a selection given, to make a bald statement of facts sound like they are writing to a friend. This section, like the rest of the book, is very nicely designed, and very accessible to the student; and the most satisfying aspect of the lessons I've taught using this book is that the students leave feeling that they've achieved something really useful.