A good provenance is not supposed to add to the value of a work of art, but it does. The information that an object was once owned by someone with famous good taste is worth money on the auction block. A collection mainly of Dutch 18th-century drawings that partakes of this quality, coming up at Sotheby’s Amsterdam on 19 May, is the Unicorno Collection, accumulated over the past 50 years by Saam and Lily Nijstad.