Michael Kimmelman writes: “The other day, I wrote about two new branch libraries in New York City, which share an unusual feature: They’re both paired with 100 percent affordable housing developments. The economics of building subsidized housing in America depends on land that costs little or nothing, which almost inevitably means building on public land. But having sold off much of what it owned, New York no longer has a large inventory of big, usable lots for deeply affordable projects. The city does own library branches, however. And they occupy public land in the heart of many neighborhoods.”