Next week, the St. Regis will unveil an “invisible” Thornwillow Press store to offer guests a one-of-a-kind retail experience befitting of the Beaux-Arts beauty. At the St. Regis, Thornwillow Press, the city’s beloved stationer, book-binder and publisher, will feel more like a residential library than a store—think sumptuous reading chairs, an on-site librarian, and a …