In my Internet ramblings, I recently stumbled across this essay written by David Widgery soon after Kerouac's death in 1969. It gives us a then-contemporary view of Jack's contributions and also mentions the famous "Joan Anderson letter" written by Neal Cassady that some say was singularly influential on Jack's style (on p. 161 of Subterranean Kerouac: The Hidden Life of Jack Kerouac, author Ellis Amburn calls this letter "the greatest literary influence on Kerouac").
If you're a true Kerouac fan, check out Widgery's essay as well as the Joan Anderson letter. They're a necessary part of your "Kerou-education."
Put Subterranean Kerouac on your list, too (most of it's available on Google Book Search).