The city’s streetscapes, though infinitely variable and always changing, are unmysterious: We walk them every day. But if looking around and even looking up have become almost prosaic, looking down at New York City still is not. Alex MacLean’s Up on the Roof: New York’s Hidden Skyline Spaces (Princeton Architectural Press) takes a targeted, bird’s-eye view to document the many uses New Yorkers find for their rooftop patches. The most ordinary-seeming roofs, he discovers, in fact function as graffiti repositories, farms, pools, restaurants, observation decks — and at the right angle, which MacLean has a knack for finding (with the aid of a helicopter), as beautiful art. Below, a slideshow of a handful of the book’s 200 rooftops.