Wharton was 40 years old when she moved into the Mount, and she stayed for only a decade, but it was a decisive one. Until that point, she'd been an unhappily married society matron who dabbled in writing. At the Mount, she became a wildly successful novelist, experienced a sexual awakening (with the no-account rake Morton Fullerton), and prepared to leave Teddy for good; they divorced in 1913. As architectural historian David Dashiell told me during the generously exhaustive, erudite tour he provided: "It was here that she realized she could control her life."

Her home's discreet unorthodoxy was a fitting backdrop for the upheaval to come. Take the tiny entry vestibule, which completely upended standard American notions of what an entryway should be. No showy portico here; instead, this fiercely private woman installed a narrow, grottolike intermediary space into which visitors could be admitted, vetted, and only then, if deemed worthy, ushered upstairs. As she wrote in The Decoration of Houses, "While the main purpose of a door is to admit, its secondary purpose is to exclude."