What are some tips, methods, facts that I need to know about starting a rooftop garden?

My wonderful girlfriend is contemplating starting a small, perhaps communal garden on the rooftop of her NYC apartment building (~30 flrs.) She is ambitious and sharp as hell, but I figured I would ask the community for some tips to get her started. I personally wonder: when the best time would be to get this started? The size, the amount of work necessary, the longevity of the project (seasonal or year-round). Then I imagine it being contingent on what she wants to grow.

I believe she wants to grow vegetables mostly (she’s a veggie), but she is probably open to other things as well, such as flowers, plants, even fruit.

Also, suggestions for books, websites, et al would be very much appreciated.