The term "stiffening" refers to the strengthening of paperback books by reinforcing the original publisher's cover at the joint and at the front and back. These books can usually be identified as paperbacks that have an additional thin white board glued to the inside of their covers, a linen strip added front and back at the inner hinge or joint area, and are trimmed down slightly from their original publisher's size.

Between the early 1990s and the advent of shelf-ready paperbacks, the Preservation Unit stiffened approximately 5,000 - 8,000 paperbacks yearly. It still stiffens some books sent to it that need a cost-effective reinforcement. These stiffened paperbacks should be sent to Preservation if they circulate and show significant or unsightly wear-and-tear upon check-in.