The book is Protocol, by Mary Jane McCaffree, Pauline Innis, and Richard M. Sand, Esq. I bought it because while researching information about flags, I would find instances of flag etiquette that cross-referenced or referred to this book.

Subtitled The Complete Handbook of Diplomatic, Official and Social Usage, I decided the book would be an excellent addition to my other reference books. First published in 1977, this 25th anniversary edition was printed in 2002.

I have not started reading it yet, but I found this in a quick glance in the chapter on flag etiquette:

The most probable story of the origin of the Stars and Stripes is that Commander-in-Chief George Washington’s personal flag, which was a blue field with thirteen white stars, was substituted for the crosses of St. George and St. Andrew in the Grand Union flag.

This is the first time I have read an explanation of the flag’s conception describe as such. I am not an expert—I research and study, and extrapolate (and quote) as best I can—but this makes a lot of sense to me. George Washington’s personal flag means his field commander’s flag, which every general would fly over his headquarters.