[Irish Wolf]

Books might work as currency, but only in certain specific circumstances. In Lucifer's Hammer, Dr. Dan Alderson, whose skillset as an astrophysicist was not itself terribly useful after Hammerfall, bought his way into the Stronghold with a collection of instructional books (his down payment was The Way Things Work, Vol 2; Vol 1 was safely secured with the rest of the books in plastic bags stuffed into a disused septic tank).