Not going to say exactly where, just in case it's still like that (which I doubt, because this is a story from the early 90's), but... When I was a teenager, I used to skip school a lot and spend time in and around the university library that was several blocks away from the high school. They had maybe four floors of books, and they were protected by the same kind of anti-shoplifting tag system they use in stores now. The basement floor had a section of the library that hadn't yet been upgraded from the Dewey Decimal System, and if you hunted around, you could find books from the 1700s or earlier.

At any rate, they had two security flaws that would have made it very easy to loot: First, they were apparently giving priority to new books while installing security tags, and the vast majority of the books in the "old" section didn't have them. Second, they'd inexplicably rerouted traffic inside the lower section such that you could get to the rest of the library via one route without being scanned, but you could pass through a scanner via another route, without ever leaving the secure area. So the scam would be to root around and find an ancient and valuable tome in the old section, exit to the rest of the library, then come back in the other direction to test whether it would trigger the alarm. If it did, you'd just explain that you were returning from one of the upper floors. If it didn't, then you could throw the book in your backpack and walk out the front door with it.

(Needless to say, I took the square route in life and never became a dark and sinister merchant of purloined books.)