When my first DADT book was self-published in 1997, I had one footnote file, divided by each of the six chapters, on which I added footnotes for readers who already had the book. At the time, the effectiveness of search engines and of "free browsing" of whole books online was not yet known. In mid 1998, I split the one footnote file into separate files by chapter, as shown.
My other two books also have footnote files (see them online at the content directory).
Original note: "The contents of this file have been moved, as follows:"