If you take a book off the shelf and do not return it within, say, an hour, you should check it out. Then people will not see that it is "available," go to look for it, fail to find it, and e-mail everyone who uses the library.
I don't mind the e-mailers. It's annoying, sure, but it's not their fault someone was a jackass.
There are friendly, helpful people at the desk. You can check things out to your carrel, or to yourself. Your library card is your ID card, so you really should have it. Also, if you end up not needing the book after all? You can just return it. Checking it out does not commit you to dragging it around on your ankle until the due date.
This is how libraries work, people! It's not hard!