Before his death in 2003, the Collectors Club of New York declared him, in 1996, to be the "outstanding philatelist of the last half of the twentieth century." Boker was able to put together some of the finest collections of rare postage stamps ever seen in philately, with his German States collection probably being his finest. He was also an expert in early American postal history, and was responsible for rewriting large sections of the Scott Standard Postage Stamp Catalog with regard to United States postmaster provisionals, locals and carriers. John Boker also accumulated large collections of Australian States, State of Buenos Aires, Confederate States of America, Fernando Po, Ionian Islands, Réunion, Romania, Serbia, and the Spanish Philippines.
His research of United States. postal history allowed him to write extensive articles on the subject.
He owned the only known surviving copy of the Buenos Aires 1859 1p "In Ps" tete-beche pair. He was also interesting in precancels of United States stamps, and had almost a complete collection.
Among other work he dedicated to the subject, he was editor of the Hoover Brothers Precancel Town List. In addition to writing numerous articles on philatelic subjects, Boker remained very much involved in philatelic activities, such as at major philatelic exhibitions, where he served as judge.
At the Collectors Club of New York he served as president and as governor.
At the Philatelic Foundation he served as trustee and as chairman of the expert committee. The sale of his German States stamp collection, from 1985 to 2000, was so extensive that it required eighteen auctions by Köhler to complete its sale. The illustrated auction catalogs of this sale represent a significant contribution to philatelic literature and philatelic knowledge.