Another good place is the library. Just be careful about which library you choose. This is a great place for talking, flirting, munching, but not for absorbing a serious academic subject. There are plenty of better libraries at Columbia, libraries at which one can actually concentrate on school work. Many upper floors of Butler offer solitude.
Unused classrooms can make a good place to study.
In an urban setting such as New York, care should be taken that the building is not so deserted as to present a risk to one's safety. With Whom It is often advantageous to study with friends. Having a friend nearby, even if studying a different subject but also intensely working, is a tremendous encouragement. The best reason for studying together for the same class is to build on the knowledge base one has already acquired by studying individually. With all participants having perviously devoted individual time to studying, everyone can receive the maximum benefit.