Although I haven’t seen a second novel, a sequel novella called “Unburning Alexandria” featured Sierra chasing around 410 A.D. Alexandria.
Ian and his cohorts have a reprise in “Ian, Isaac and John” (Nov 2011), where a descendant of David Bowe comes back to 1975, purportedly to improve the mix on a Bowe track, but quite possibly with additional motives involving John Lennon. And there are more stories to come, all in Analog.