An Investigation into the Myth-Patterns of Latter-day Primitives with Especial Reference to the R’lyeh Text is a fictional scholarly work created by August Derleth, attributed to his character Laban Shrewsbury.
It first appeared in Derleth's story "The House on Curwen Street" (1944) and is also mentioned in his The Lurker at the Threshold (1945), "The Gorge Beyond Salapunco" (1949), and "The Keeper of the Key" (1951).
The book also figures in Lin Carter's "The Horror in the Gallery."