I'm looking for a good reference for learning about Hasse-invariants ($p$-ranks) for curves of arbitrary genus over a field characteristic $p$.
All the usual suspects I've searched (Milne's Étale cohomology, Mumford's red book, Hartshorne, Silverman) either omit the topic entirely, give a definition only for elliptic curves, or assume that the reader is already familiar with the Hasse-Witt matrix. I didn't see anything in the index of SGA 1 either, but perhaps somebody more familiar with SGA (or with a searchable copy) might be able to find something.
Any suggestions? Thanks.