Little, however, of the wall was seen, for it was nearly hidden by the arched book-cases; and the ponderous tomes, mostly bound in black or white vellum, long since grown dingy with age, contrasted forcibly with the gayer ornaments of their habitation.
Source: wiktionary
From this list are purposely omitted books printed upon vellum, Books of Hours of the Virgin Mary, and illuminated books; for these are rarities within reach of the wealthy only.
Source: wiktionary
The metaphor is apt, imagining one's spiritual life as a poem, an authored thing, and not preserved on vellum either, but open to revision.
Source: wiktionary