the folklore index Coover used

Coover used to have his students use the Motif-Index of Folk Literature, by Stith Thompson. The full title is "Motif-index of folk-literature; a classification of narrative elements in folktales, ballads, myths, fables, mediaeval romances, exempla, fabliaux, jest-books, and local legends."

The index is a categorical compilation of common and uncommon motifs from a wide range of literary traditions and genres, spanning thousands of years and many cultures. Each entry will name a motif, such as magic carpets, and list occurences of that motif in the literature covered (it directs you to an entry in some bibliography of folklore, usually). It was originally published in 1932-1937. A revised and expanded edition was released in 1955.

The work is a highly regarded and well-loved resource among students of folklore -- definitely the gold standard for reference works on this sort of thing, of which there are quite a few (Motif-Index of Early Irish Literature; Motif-Index of Polynesian, Pelonesian, and Micronesian Narratives, etc.). All the more specialized indexes use Thompson's system of organization.

* Thanks to Ben Wiebracht for researching this reference work.