There are few lines in New York City more powerful than “Come check out my roof.” The promise of a rooftop viewing prolongs mediocre dates, ensures attendance at impromptu parties, and solidifies connections among neighbors who would otherwise prefer to have nothing to do with one another. (“Shall we have a beer on the roof?” “Why not!”)

Even an unfinished, borderline dangerous rooftop can dramatically alter one’s life in positive ways, connecting a claustrophobic ( “cozy!”) studio apartment to a sphere of possibilities accessible only to richer neighbors: gardening, sunbathing, hosting dinner parties, smoking-without-suffocation, flirting with a view, adequate AT&T reception.

The only thing better than possessing one’s own roof is visiting a neighbor’s roof. Because ultimately we all are desperate to know: What’s up there?! This is precisely why Alex MacLean’s Up on the Roof: New York’s Hidden Skyline Spaces is utterly addicting. It answers this question for 183 buildings across the city, many of which we are not likely to garner an invite to. (The mini-forest on Bette Midler’s Upper East Side 3,100-square-foot rooftop space, for example, or 77 Water Street, with its model of a World War I British fighter plane. ) Often what’s up there is not what we’d expect from below: Playgrounds, pools, beautifully landscaped gardens, and messages to aliens above pop up in surprising places throughout the city.

MacLean, a pilot and photographer, has long been known for his incredible aerial photographs. For this project, he flew low around the city in a helicopter, making his way around skyscrapers and wires. (Someone else was piloting, don’t worry.) Occasionally you can sense that people know he is there, as when a beer drinker waves from atop the Standard.  Other times, people don’t know or don’t care: A topless sunbather seems unperturbed, and a bride chats up her guests, immune to yet another photographer.

Roofscape makes up “about one-third of the impermeable surfaces of Manhattan,” the introduction by architectural critic Robert Campbell explains, referring to surfaces that don’t let water through at all, like streets and parking lots. (A park, on the other hand, is permeable.) In the text between each section, MacLean builds the case for thinking more strategically about rooftop spaces. White roofs help cool the city, while dark roofs collectively raise the ambient temperature of the city by as much as 6 degrees. Roof gardens provide a host of environmental and community benefits. So why aren’t there more?

MacLean’s work often draws its power from the way he discovers patterns on the earth’s surface, capturing monumental forms hidden in everyday places—the beach, a field of ice, a housing complex. Perhaps because MacLean is making such an effort to get up close and show us the context for these roofs(the side of the building or the street below), we get less of his mind-blowing sense of composition in Up on the Roof.  Stunning shapes are replaced by a kind of benign voyeurism. Still, the photos will inspire city-dwellers to reconsider how they’re using the space above their heads. And that, perhaps, is MacLean’s point.

See all the pieces in the new Slate Book Review.