In fact I do understand you and in some places throughout the book i have felt that he leans towards working off a diary with dates, etc. too.
And although OF COURSE I have not only never been there but also was born much too late to could have been there at all if I had just traveled there I approached the book from a totally different angle and like it just for what it is (to me, anyway):
1) It adds to the documentation of post-war jazz in Britain and fills gaps (stretching beyond the regional aspect of the book), including in its illustrations, that I feel won't be filled elsewhere - particularly since it adds to a lot of ground not covered by Jim Godbolt in HIS history (which I feel is nice for lack of a better one but I find it rather unbalanced and often scratching the surface only)
2) It essentially covers a key period of jazz that I am interested in and therefore avoids turning into a "history" book that purports to cover it all but ends up focusing too much on more recent decades because more recent material is easier available and researchable there and likely sales propsects are better if you mention artists still on "everyone's" minds
3) A strictly personal thing: I like those regional histories that IMHO flesh out the overall picture. Anyone can cover New York, London, as the no. 1 focus points and leave it at that, but ... I for one find lots of interest and merit in regional books on Detroit, Chicago, Newark, Boston, etc. I have bought over the years.
Highly subjective, of course, and I would not expect too many others to attache the same kind of importance to those criteria.
BTW, I would not consider it "essential" overall either but rather a very good "special interest" book.
OK