Needless to say that JB Gillespie was a major contributor to jazz history. Period.
Dizzy lived long enough to tell the story of bebop and its inceptions (not Charlie Parker, Bud Powell, etc) and this auto-(with Al Fraser) biography is a vehicle for reaffirming his role and, at the same time, claiming the status he thinks he deserves as a champion of modern music.
As in fact it seems he needed to assert it all in order not to be forgotten and, at the same time, show the world that the "jazz community" highly valued his contribution. That said, the writing (full of witnesses, from John Lewis to Kenny Barron, e.g.) sometimes gets boring, repetitive (some of the concepts are repeated two or three times, like "the drums were taken from slaves in Northern America but not in South"), shallow and too much of a laudatory thing. Pinched with anecdotes that don't really add up, honestly, it is hard to reach the 500 pages having read them all.
The book's structure and length is a metaphor of what happened with John Birk's life as an influential musician: having already become a celebrity in the revolutionary 40's, from the 50's on he got stuck on his business and musically speaking became irrelevant until the end of his days (minus the legendary status). A bunch of contemporary trumpeters (Miles Davis, Clifford Brown, Lee Morgan, Freddie Hubbard) that were looking ahead of their time surpassed him in success and creativity and that is probably why Dizzy keeps referring to bebop and Charle Parker (the "good times") constantly. So, ironically, the first 60 pages are focused on Gillespie's youth (1917-1937) the following 240 are devoted to the period 1937-1947 (which is a great treat) and the remaining 200 (1947-1977) to "the rest of his life". It is those last 200 pages that are hard to chew, swallow and digest, except for the chapter "Evolutions" that contains a synthesis of what is stated before in regards of music evolution.
Periods of his life are ignored, certain characters are not mentioned (Howard McGhee?), the overall priority seems to be given to certain aspects of his personality and certain achievements more related to his post-humous glory than to music's glory.