Most people would agree that doing technical work, whether it be pure or applied, and learning the background knowledge necessary to do this work comprises most of the literature and curriculum in mathematics. What is almost never discussed however is how to communicate mathematics in a way which is as clear as possible to as many people as possible. Writing technical articles is a domain of its own, and one which many mathematicians and applied mathematicians learn from their peers and advisers, as well as here on Math SE (e.g. How to write a good mathematical paper?). There are many templates, good general styles, and even specific styles which should be used for different journals, and over time it is typical that any successful mathematician will become proficient in this area.
A topic on which there appears to be no guidelines from any one of these groups however is textbook writing. In fact, I don't even recall ever having a serious discussion with anyone in any technical field about what even makes a good textbook good! Of course everyone knows the texts they like, and different people prefer different texts for different reasons, but apart from personal opinions, I wonder if is possible to catalog a few strategies and approaches to good textbook writing with which a majority of people might agree.
FYI: Currently I'm trying to think out the best way to write a concise, yet complete, introductory control theory text. Most of the existing introductions are 500-800 pages, mash all sorts of different approaches together, and often leave 3rd year engineering undergraduates thoroughly confused and enraged with the subject. My dream is to write a text which can explain introductory control theory in as lucid and interesting a way as David Griffiths' Introduction to Quantum Mechanics explains the eponymous subject at the undergraduate level.