This is a list of stories that can help you to understand the CthulhuMud. They're also good reading :-) All stories are short stories, except for the few that are marked as novellas. All of the listed stories are by H.P. Lovecraft.
The Dream Quest of Unknown Kadath (novella)
The Shadow Over Innsmouth
The Call of Cthulhu
Imprisoned With the Pharaohs
The Horror at Redhook
At the Mountains of Madness (novella)
These are some short stories are related to the dreamlands, but can be rather hard to find. They can all be found in the collection Dreams of Terror and Death, ISBN 0-345-38421-0.
The Cats of Ulthar
The Other Gods
The Silver Key