By Peter Dickinson.
The sequel is Angel Isle, but I haven't yet read it.
This is a straightforward quest fantasy. Straightforward isn't meant to imply that there are magical swords, elves, or dwarves, but more that everyone is who they appear to be. No one is on ambiguous moral ground here, at least from the perspective of the protagonists, and they never trust anyone that it turns out they shouldn't have. While fairly enjoyable to read, I don't think there is a lot of real depth, although some is hinted at, such as the Emperor's policies concerning life and death. (If you die without paying your death tax, your progeny is liable to be enslaved and sold to pay the debt.)
The story principally follows four people from the Valley, a region that has been protected from the Empire and from barbarian tribes to the North by a magical barrier for the last two hundred years. When the barrier starts to fail, Tilja and her grandmother and Tahl and his grandfather set out to find the man who initially put it into place, planning to ask him to restore it. Along the way, they learn about the Empire that they've been isolated from for the last two hundred years.
Enjoyable, but straightforward, making only relatively minor references to issues like aging, death, and the corruption that comes from power.