Essentially, the world of Serpent's Blood is a lost human colony, dumped on a planet where corruption and corrosion destroy metal, stone, and just about everything else. In the millenia since they landed, the colonists have lost all their advanced technologies to the ravening moulds and bacteria. Only their adapted human flesh is able to survive the depredations.
All knowledge is passed down by word of mouth, committed to fallible memory.
The humans have spread out over the planet, forming themselves into small kingdoms. By various roundabout means, the two find themselves on a quest of sorts, along with a band of other assorted soldiers, merchants, thieves and misfits. Something is happening to disturb the perilous equilibrium the humans have built up on the planet, and they have to find out what it is. By the end of this volume, a number of things have been resolved, and the company is assembled for the further adventures and wonders of volumes 2 and 3. I see no reason to assume he won't do the same with The Book of Genesys.