A large library with plenty of space to work. Ther...

A large library with plenty of space to work. There are many desk workplaces without sockets on the ground floor and in the front area of the 1st floor (level 2) in the back area of the 1st floor there are tons of workplaces with sockets and extra light. There is generally enough light. So far, my impression is that you will definitely get a seat as long as you get there before 15:00.
Unfortunately, the windows are not very well sealed against sound, which means that some ambient noise gets into the interior of the library, so if you are sensitive to something you should bring earplugs, earmuffs or noise canceling headphones.
In general, I think that the atmosphere is very pleasant. There is a large balcony where you can smoke and an area to eat. You can also buy various snacks and drinks, but you are still in Paris and the prices are correspondingly high. Otherwise there is a small Carrefour directly opposite.
The toilets leave something to be desired (no soap, hardly any pressure on the taps, often long queues in front of the ladies' room. I only noticed when I was walking by!)
So far, I haven't had to pay an entrance fee, but haven't found out whether this is the case at certain times. I've always arrived before 2pm. Of course you can stay as long as you want.
There is also WLAN, which does not require registration, but it is limited to 500MB, which is sufficient for most things. Otherwise there are also PC workstations with unlimited internet access.