An Investigation Into the Myth-Patterns of Latter-Day Primitives with Especial Reference to the R'lyeh Text

This book was written by Dr. Laban Shrewsbury and published by Miskatonic University Press in 1913 (and possibly again in 1936). It describes the rites and centers of the cult of Cthulhu.

Game stats for this work may be found in the Keeper's Compendium Vol. 1. (Much thanks for this tip goes out to The Man in Black)