When Stephan Janson opens the door to the densely saturated wunderkammer that is his Milan home, the only sensible response is to gasp. A taxidermied alligator stares stiffly at you, mounted on a 17th-century gilt wood settee of ripped silk, bought for a pittance from an antiques market. A full-wall vitrine encloses a riot of feathers: African and Amazonian headdresses, Native American war bonnets, Chinese hairpieces. Floor-to-ceiling shelves of books—part of the apartment’s 25- year-old redesign by Roberto Peregalli—line every available wall, organizing the more than 20,000 volumes by art, history and other subjects. Hand-knotted carpets conceal the wooden floors, decorate walls as fragments in frames and even hang on curtain rods, shrouding the space in near darkness and blockin...