Alan Dean Foster 
Voyage to the City of the Dead [EPUB ebook] 

Far, far Upriver, where knowledge failed and fable ruled, in a frozen land of demons and monsters, the City of the Dead stood guard over a great treasure. Now the Mai, Delta-dwellers of the planet Horseye, as befitted a race of busy traders and merchants, had a great respect and love for wealth. But, a prudent species, they knew also that danger was, well, dangerous. So their thoughts turned to the Humanx couple, Lyra and Etienne Redowl, impatiently waiting permission to explore the planet. Suddenly bureaucracy was benign, permits permitted and their epic voyage up the most spectacular river chasm in the whole Humanx Commonwealth was underway…