I recall seeing the term "jam band" in some older articles from Rolling Stone and other places, in issues from the late 60s. But it didn't describe a genre, the term just described a generic approach to playing live rock music. The first time I recall seeing the term "jam band" (or one word: "jamband") to describe a genre was around the time the Blues Traveler had their first rock radio hit ("But Anyway") in early 1990 or so. I can't recall where I saw or heard it but I definitely recall Blues Traveler being on the forefront of....something before Phish got much in the way of national press

I haven't read the book, but I sure hope the author finds space in it to devote to Max Creek, who are SUPER-important in the evolution of the jam band genre but who rarely get any credit