The Cassini Division (Fall Revolution) by Ken MacLeod

Ellen May Ngewthu is a soldier and leader of the Cassini Division, the elite defense force of the utopian Solar Union. Here in the twenty-fourth century, the forts of the Division, in orbit around Jupiter, are the front line in humanity's long standoff with the unknowably powerful post-humans -- godlike and remote beings descended from those men and women who transformed themselves with high technology centuries ago.