Neinken was primarily interested in 19th centuryUnited States postage stamps. He supplemented Ashbrook's work by publishing “The United StatesTen CentStamps of 1855-1859” in 1960 and “The United States One Cent Stamp of 1851-1861” in 1972. Neinken also wrote “United States: The 1851-57 Twelve Cent Stamp ” a work for which he received numerous honors and recognition.