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Be it known that I, WILLIAM STEBBINS BARNARD, of Canton, county of Fulton and State of Illinois, have invented a Book-Support, of which the following is a specification [of] this invention [that] relates to a support or holder for books, engravings, photographs, cards, and other things, which it is desired to stand on edge, and retain upright on flat surfaces, as on tables, shelves, &c. The invention consists in a book support or holder composed of two unconnected angular end pieces to be placed at the two extremities of the row of books to be kept upright, each being adapted to receive and to be held in position by the books set upon its horizontal extension or base, and to support, by its vertical extension or side piece, the outermost volume at its end of the row, being freely movable toward or away from its fellow, in order to accommodate the support or holder to. any number of books.
Patent US 186974 A, "Improvement in book-supports", published Feb 6, 1877. (So we've missed "Book End Invention Day" this year - mark it on the calendar for next!)
It's amazing how many elaborate and complicated book shelving solutions were patented before this elegant and simple solution showed up.