The best training can be given through mentoring from more experienced professors. There is no universal rules of success for writing good textbooks. Also, you have to keep in mind that writing a good textbook is a cultural context issue since the style use in one country might not be useful in another country. For instance the American style used in textbooks could not fit very well the Japanese style or the German style.
Another issue is related to the level of education, i.e. high school or university, and within a university undergraduate or graduate level. Also the domain of education. For instance, it is one way to write a textbook of mathematics for engineers and another way of writing the same textbook for students in mathematics. The language and degree of logic are different.
In conclusion, define first the goal of your textbook and the category of readers that book addressed. Then consider the degree of knowledge the potential readers may have in that field to come with your explanations at that level. The content should be measured in concordance with the importance of the discipline within the curriculum. The style should be clear, logical and above all stimulating. The main purpose should be to overcome the knowledge transfer barriers and to stimulate students to reflect upon the topics. A good textbook should invite students to be active readers and offer them a critical thinking approach. Also, a good textbook contains successful stories and case studies in the field of that discipline.
i agree with prof Constantin Brattain, there should be personnel mentor who should guide and provide useful information to the staff who needs training. Textbooks contains established knowledge that need to be transferred to students.