Stafan Zweig (writing in 1929, Vienna) in Lapham’s Quarterly:
I reiterated my object in consulting him: to get a list of all the early works on animal magnetism, and of contemporary and subsequent books and pamphlets for and against Mesmer. When I had said my say, Mendel closed his left eye for an instant, as if excluding a grain of dust. This was, with him, a sign of concentrated attention. Then, as though reading from an invisible catalogue, he reeled out the names of two or three dozen titles, giving in each case place and date of publication and approximate price. I was amazed, though Schmidt had warned me what to expect. His vanity was tickled by my surprise, for he went on to strum the keyboard of his marvelous memory, and to produce the most astounding bibliographic marginal notes. Did I want to know about sleepwalkers, Perkins’ metallic tractors, early experiments in hypnotism, Braid, Gassner, attempts to conjure up the devil, Christian Science, theosophy, Madame Blavatsky? In connection with each item, there was a hailstorm of book names, dates, and appropriate details. I was beginning to understand that Jacob Mendel was a living lexicon, something like the general catalogue of the British Museum Reading Room, but able to walk about on two legs.
…True, this memory owed its infallibility to the man’s limitations, to his extraordinary power of concentration. Apart from books, he knew nothing of the world. The phenomena of existence did not begin to become real for him until they had been set in type, arranged upon a composing stick, collected, and, so to say, sterilized in a book. Nor did he read books for their meaning, to extract their spiritual or narrative substance. What aroused his passionate interest, what fixed his attention, was the name, the price, the format, the title page. Though in the last analysis unproductive and uncreative, this specifically antiquarian memory of Jacob Mendel, since it was not a printed book catalogue but was stamped upon the gray matter of a mammalian brain, was, in its unique perfection, no less remarkable a phenomenon than Napoleon’s gift for physiognomy, Mezzofanti’s talent for languages, Lasker’s skill at chess openings, Busoni’s musical genius. When someday there arises a great psychologist who shall classify the types of that magical power we term memory as effectively as Buffon classified the genera and species of animals, a man competent to give a detailed description of all the varieties, he will have to find a pigeonhole for Jacob Mendel, forgotten master of the lore of book prices and book titles, the ambulatory catalogue alike of incunabula and the modern commonplace.