Part 2 of Jill Williamson's Epic Fantasy Series The Kinsman Chronicles
With Prince Wilek still searching Armania for a murderer, his brother Prince Trevn finds himself pulled deeper into the dangerous politics of who will be Heir. Trevn, though, is far more interested in his studies with a local priest and the attentions of a young honor maiden.
The book that he has been copying during his lessons is not the book that teaches of the god of the soil. Instead, it teaches of a one true God. If accurate, then it means there is no soil god to satiate. There is nothing they can do to save their world, and all of King Echad's sacrifices have been futile.
The Heir War is collected together with parts 1 and 3 in King's Folly.