It's a Brit thing in another sense, Larry. Unlike you, Cook and Morton as commentators on the 20th century American music called jazz lack two factors: not American and too young to be around when much of it happened - hence the emphasis on records.
There's a fallacy in that idea, Bill - the idea that someone present in an historic period will have a greater understanding than someone looking back to it from much later.
But someone present in that historical period will have a perspective that others will never have.