 This edition was also published in paperback format (most notably, a two-volume mass market paperback edition); it is still widely available in used bookstores.The 1964 edition was also released as a single-volume comb-ring bound paperback mass-market edition starting in November 1973 and continuing into the early 1990s.The 1975 edition was the last to be edited by Becker and remains the most popular, with more than 6 million copies sold.