So, do y'all want to know the real answer? The answer is, it depends ...
just so that you're aware, the books that you see on public library shelves normally represent about 1/3 of what the library holds in the circulating collection; the amount of shelving available reflects that. At most libraries, we literally could not re-shelve the entire collection if it all came back at once. Therefore, the due date is normally a suggestion, mostly meant to remind you that it is a loan rather than a gift. We also do not expect the normal fines to serve as much of a deterrent to late returns; fines are an additional source of revenue, albeit a very small one. (Most public libraries still charge somewhere between .10-.25/day for late fees, and often have grace periods as well; we are not raking in a fortune in fines.) At most libraries, waiting a bit on declaring it lost and billing for replacement cost is deliberate; we don't particularly want patrons to get into the habit of using our collection as a "try before you buy bookstore", especially if the title in question is out-of-print and cannot easily be replaced.
Where does it depend? Best-sellers and books needed to support K-12 school curriculum. Not all public libraries officially support school curriculum (which is a funding issue of library vs. school board), but those that do seldom have enough titles in the collection to do it in a meaningful way. However, when it is science-project time in an entire district, and every science-project title is checked out by the day after the deadline is announced, then yes, PLEASE bring those books back as soon as possible, because you know that there are other parents whose kids need them just as much as yours do. Make copies of the important pages and get the book back into circulation as soon as possible, please.
Best-sellers normally are provided by public libraries through rental programs, in that the library pays to rent multiple copies from a book wholesaler rather than purchasing them. Demand for best-sellers is transient, and it isn't the best use of tax dollars to buy 30 copies of a book like Gone Girl when the demand is going to slack off within about 6 months of publication. We usually buy 2-3 copies for each branch, but rent additional copies for a few months to meet initial high demand. After the rush is over, those go back to the wholesaler in trade for extra copies of the next hot title. (We normally can't use this solution with things like the science-project books, as those are not volume-discounted by the publishers the way that best-selling titles are.) We do like to keep our spend as low as possible on the temporary rental collection, so we often will push a bit harder to get those back as quickly as possible and into the hands of the next person on the waiting list.
By the way, the electronic best-sellers that libraries provide also cost us a bit more, which is why we seldom have a really large number of copies available. Public libraries almost never own popular electronic titles outright; instead they pay to license them from for-profit service companies. The hotter the title, the more the service charges for each "seat" license; which translates into how many simultaneous check-outs it may have, or in some cases, how many check-outs it may have, period, before the library is required to buy it again (e-books, it seems, "wear out" a whole lot faster than paper copies; at least according to certain publishers.) For titles that are not in really high demand, we often can buy access to large sets at far less cost (which is how all those Harlequin romance e-books appeared in ma ny library collections; Overdrive sells that access for a pittance compared to works from marquee publishers like Simon & Schuster.) Even now, most library budgets have funds for ebooks separated out from funds for hardcopy titles, so if you want more ebooks (or audio books), borrow more of them. Libraries need proof of demand to justify shifting funding to that service.
I constantly return things late, even if there are holds. And no I don't feel that bad about it.
