Books
Writing books or Notes is hobby as well as profession. Writing books is a skilled work, ranging from selection of topics, words, language, explanation, graphics etc. This is never ending process. A book or notes written once, requires periodic updates and modifications as reader's responses.
A writer may write a book belongs to any stream of study. One should write one's experience, knowledge, skill etc. This helps others to gain precious knowledge.
A best teacher is not necessarily write best book. And it is not necessary that a best book teach other in best way. In my experience, as I feel, there were 1000 of micro controller books, but I understand micro controller from a presentation of 400 slides.
This is why, I recommend everyone that they should compose their knowledge and skills in a book form.