Any teacher knows that the best way to learn something is to have to teach it. I've taught college writing for three years, where I've had the students create their own composition guide. They love the assignment, since it combines their need for practical results with their intellectual work in the class. They Web the result. A few examples, here.
Many students assume a consumer-vendor model of education, and resent blurring that line.
In many subjects students will arrive with knowledge, experience and intuition that is highly relevant. So a good starting point could be making a first draft of the book without reference to any sources 'out there'. By the time students eventually meet the 'experts' there is less chance they will fall back into Bryan Alexander's consumer-vendor model of education. The 'experts' will be saved from having to deal with questions that the students already 'knew' the answers to - making it a much more 'learningful' experience for the experts too. Once I was teaching a group of educators how to set up school-community partnerships.