Perlmonks successfully facilitates human relationships, people get to know and (insert verb here) each other. Each lender can be their own librarian, choosing who they will lend what to under which circumstances.
Anyone who offers or needs books could construct a list on their homenode using some keyword or phrase. Using Super Search to search homenodes (by checking the users box) one could quickly locate monks who offer or desire books. Then either group would know who to /msg with an offer or request.
Some sell, others just give them away. He puts a "From the library of:" sticker inside, and stamps them with return due dates. She makes chain books by signing inside the cover with a little note asking that it be signed and passed on when sufficiently absorbed.
Super searches that come to mind resemble "books-needed learning perl" and "books-offered mastering expressions".