It is one of the most inspirational essays I have read. Moreover, I believe it contains the answer to the question of who should write these software handbooks. People in each of these groups have valuable contributions to make, and it would be a mistake to discount any of them.
Recent retirees bring years of experience, insight, and reflection to their writing.
University professors bring a full awareness of leading-edge research. Seminar instructors have a chance to test their material in front of hundreds or even thousands of students before publishing their material in book form. Consultants see dozens of clients a year, and their observations can be based on an incredible breadth of exposure to effective and ineffective software practices. But I think that developers working on production software must shoulder the primary burden of creating these handbooks. A thinker is someone whose sole function is to think.