Rickey Vincent's Funk: the music, the people, and the rhythm of the one is a decent history of funk that has a lot on p-funk and hey wait a minute i never noticed this, a foreword by george clinton., so you can tell where Vincent's coming from
greg tate wrote some good stuff on Clinton et al in his essay collection the Fly Boy in the Buttermilk
there's also some good far-out stuff on P-funk (and pedro bell--did i get the name right--'s artwork) in Kodwo's More Brilliant than the Sun
also a good essay by John Corbett in his essay collection (title i forgot) which compares Clinton with Lee Perry and Sun Ra