He prided himself in being "the only bookseller in history...to have owned a Gutenberg Bible and the Psalters of 1457 and 1459[2] simultaneously," stressing that "'own' here is the correct word, as they were bought not for a client's account but for stock.
After working for R. Lechner in Vienna and Ernst Wasmuth in Berlin, he started his own rare book business in 1932, which prospered despite the Depression.
Among his most important sales were the Anhalt Gospels,[8] the finely illuminated Hours of Catherine of Cleves now reunited with its other half at the Morgan Library, the Arthur Houghton copy of the Gutenberg Bible for $2.5 million,[9] three copies of Caxton's first edition of the Canterbury Tales,[10] and the original manuscript of the proclamation of the Louisiana Purchase, signed by Thomas Jefferson,[11] He also purchased the enigmatic Voynich manuscript in 1961 for $24,500, and after seven years of unsuccessfully attempting to sell it for as much as $160,000 ultimately donated it to the Beinecke Library at Yale University.
[14] Early in his career, Kraus initiated a practice of buying up entire libraries or collections at a discounted price and then selling the items individually or in smaller groups, carefully researched and catalogued, for a great profit.
[19] Despite the definitive proof to the contrary, Kraus still professed that, "Speaking for myself, I believe that the Constance Missal is earlier than the Gutenberg Bible.
He also lamented a number of instances where he narrowly failed to secure some book or collection including some of the Dead Sea Scrolls,[25] or where he sold a very rare item too cheaply.
[27] Kraus also put together a collection of important manuscripts concerning colonial Spanish America, particularly Mexico, including a letter from Amerigo Vespucci.
[29] In 1947 he opened a second business in New York, Kraus Periodicals Inc., to specialize in the sale of runs of scholarly journals, and soon made an en bloc purchase of over 300,000 issues.
[33] Kraus died on November 1, 1988, in Ridgefield, Connecticut, after which the business was carried on by his widow Hanni and their daughter and son-in-law, Mary Ann and Roland Folter.