I'm not IN anymore, so i don't know if she';s underrated. But she is fantastic. I don't read much spy/paranoia politics/conspiracy fiction, because it makes my head hurt. But when I want some, I turn to Deighton, old Le Carre, and now Moriarty.
In her Spin series, humanity has mostly left earth due to ecological deterioration - the only people who've stayed are those who have religious-spiritual connections to specific places.
There're artificial intelligences, planets being raped for resources, a lot of religion/spirituality discussion/commentary. A lot of whatever it is to be human.
Spin State, Spin Control and Ghost Spin. Also her YA fiction, The Inquisitor's Apprentice.