A readable concise guide for assembling a textbook from an active Commissioning Editor, Writing Successful Textbooks is the most accessible guide I have found to publishing in this field, as well as being relevant to many other forms of publishing.
Anthony Haynes has a clear, unfussy writing style and concentrates on the practical aspects of pitching your work in the form most likely to attract an Editor. Common errors and pitfalls are highlighted, as are hints on presentation and structure. There is also an excellent chapter on electronic publishing. In contrast with many other "How to" books the author uses real examples as case studies, which I found particularly useful.
Writing Successful textbooks will be a welcome read for anyone seriously intent on publishing successfully in what is a competitive and potentially lucrative market.