The Book of Skaith, volume 3. Serious spoilers ahead.
In fact, it's a bit deus ex machina, the opening, where the starship captain they had deal with at the end of Hounds turns treacherous. Thus, among other things, sticking Ashton back on the planet, and letting Stark know that no help will be coming.
The starship captain is extracting information from Ashton about what he can loot. That Stark, under drugs, reverts to the language of the aboriginals who raised him cramps this, but he lets Stark regain his senses, and after some discussion, Stark starts to explain how to raid the House of the Mother, which lets him near enough a lamp to escape.
And then they must forge onward in a desperate and bitter land. It involves another prediction from Gerrith, a boy known as the Bridegroom of the Daughter; that not all of the Children of the Sea-Our-Mother can live in the sea; a temple where there was a massacre, but they feel less pity for the priests after their search; a people who pray every night that the sun will never rise again, and practice the ritual Death of Summer, which takes a while because it is a wicked season; a sea voyage, and more.