In Brussels there is a library that’s “open” as late as 22:00. There’s an after hours program where you register for after hours access, sign an agreement, and your library card can be used to unlock the door. Staff is gone during off hours but cameras are on. Members are not allowed to enter with non-members (can’t let anyone tailgate you incl. your friends).
I was only there once or twice in off hours. I think I was there once on a Sunday (normally closed all day so only open to after hours members) and once in the evening. It was quiet as I recall but I guess I’ve not made use of it enough to have an idea. It’s not overly busy in the after hours.
W.r.t. alcohol, the rules forbid eating and drinking in the library, but water is exceptionally allowed. I don’t know if they review the video without cause, but if someone breaks the rules, their after-hours access is terminated.
Funnily, we just discovered that one of our city-libs iss actually doing the exact same thing now too. As an experiment.
And it seems they don't have to deal with it. Quote: "i guess the library-folk is just a quiet and peaceful folk". So, no problems and people love it.
My local library has 24h access for students to a special area designed for the purpose. During the day the same area is accessible to the public. It's just a nicely laid out area with desks and chairs and a great view of the park.
It's unbeatable at 5am with a stiff cup of coffee.