tells us that, according to a defaced sign, we should not put books on elves
. (This writeup originally appeared there.)
Another reason not to put books on (sh)elves, in all seriousness, is that a library lives on statistics.
Sometimes a book will be checked out, and this is good because (among other reasons) it shows up as a data point. But books are much more often consulted and then returned to the (sh)elf, and this completely escapes the library's statistical report -- their bargaining tool to avoid a slashed budget.
To avoid this underreporting of usage, libraries will sometimes get their employees to scan the barcodes of all the items that have been used but not checked out. Thus if you were to put the books onto the (sh)elves yourself, you'd actually be annoying the library staff even though you know how to do it properly.