The cafe should be accessible from outside.
If the vendor operating the café/coffee bar proves catering for events occurring elsewhere in the library, adequate space for preparation, as well as easy access to elevators for moving food, trash, etc. is key.
It should be clear that the cafe is a retail establishment and not open seating for library customers who haven't bought anything. it also shouldn't be so comfortable that people stay in the cafe for hours.
Cafe space in the library should engage local businesses. What if it became an incubator space for pop up coffee shops and bakers who don't have their own brick and mortar locations? Or even for popular food trucks to have a set location/residency at the library.