How Important Are Library Sales to the University Press?

In the comments section of a recent Scholarly Kitchen posting by Rick Anderson, a now-familiar point of controversy was raised: to what degree do university presses rely on libraries as customers for their books? It's a commonplace assertion that, contrary to longstanding popular belief, libraries are not in fact the primary customers of university presses, and this assertion was made again in the comments. Rick expressed his belief that while this is true of university press publications generally, it's probably not true of scholarly monographs specifically, and that the decrease in libraries' share