Vellum

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.

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.

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.