As a student in Paris, I worked either from home or in the cafes. There were cafes where I went to socialize after class or to work in group, or cafes where I would go to work alone.
However, many of my friends spent days and nights in the Library. The famous library for students in the Latin Quarter (where the Sorbonne and many of the lyces with prep classes are) is the Sainte-Genevieve Library. It's open Monday to Saturday (10:00 am to 10:00 PM) except Sundays and holidays. I went on their website and they now have a system that allows you to see how many seats are left in their 3 reading rooms.
We often had conversations with my friends who spent their days at Ste-Genevieve as they did not understand why I'd rather work from home or in a cafe... It was not so much for the access to books, although they did borrow or look at books. It was more for the atmosphere: silence, but also other people's working around (maybe a form of social pressure? you have to work as others work around). I also remember groups forming and chatting in front of the library (taking breaks, drinking coffee, smoking, etc.). Sub-communities based on your school or university existed but it was also the place where they mixed. There was also the symbolic value of Ste-Genevieve an old library (it was founded in the 6th century and inherited the writings and collections of one of the largest and oldest abbeys in Paris; the current building was built in the 19th century and is described as a major cultural building). Founded in the sixth centuryin the heart of Paris, next to the best schools and a major university, La Sorbonne.