I've been making some progress at last with my book on Knight's Tours. I have masses of material but the problem has been organising it in some sensible manner. The trouble is that putting things into historical order starts off quite well but eventually runs into the sand. On the other hand starting from first principles and putting everything in logical order means that the history can only be accessed via a chronological appendix that refers back to the appropriate pages.
The solution I have found is to use the historical sequence as the main structure, providing a series of themes, and to put in more recent developments where each subject first comes up, resuming the historical sequence at the start of the next section. So far it's working out quite well. It produces a series of Sections which I letter A, B, C, etc, with Chapters numbered 1, 2, 3, etc within each Section. A Chapter being devoted to a single narrow subject and covering on average six or eight pages (A4, 11 point text, diagrams with cells 1/5 of an inch wide).
I hope it will find a publisher, but if not I can put it on the web in the form of a series of pdfs. But there's still along way to go.