The Light Fantastic by Terry Pratchett (1986)

This is the sequel to The Color of Magic and picks up where the previous book left off. It opens with the Octavo saving wizard Rincewind, Twoflower and the Luggage, after they fall from Discworld. 

The Call of Cthulhu by H.P. Lovecraft (1928)

This is the quintessential story on which Lovecraft's Cthulhu mythos is built and the one on which Bel-Shamharoth (Part 2) is modeled. The story can be found in virtually every collection of Lovecraft's work and from Project Guttenberg, as Lovecraft's stories have recently come into the public domain.

Conan: The Thief, The Conqueror, The King: The Collected Adventures of the World's Greatest Barbarian by Robert Howard (2020)

These collected stories were originally published in Weird Tales magazine between 1933 and 1936. Conan the Barbarian is the basis for Hrun the Barbarian, who appears in Parts 2 and 3 of The Color of Magic. Robert Howard also wrote horror stories in the Lovecraftian genre, making Hrun a perfect match for the Bel-Shamharoth story.

The Adventures of Fafhrd and the Gray Mouser by Fritz Leiber (1939-1988)

A series of stories following the eponymous Fafhrd and the Gray Mouser that were published over a 49-year period. Lieber created Fafhrd and Mouser as a foil for less realistic characters like Conan.