This is a collection of things that cannot be. It has been described as an exhibition of imaginary books, a bibliophilic entertainment, and as a post-structuralist conceptual art installation. It consists of the physical instantiations of Lost Books (with no surviving example), Unwritten Books (planned but unfinished), and Fictive Books (existing only in stories in other books). An amusing video presentation on the collection can be found at the Princeton Bibliophiles. (The talk begins at 3:15 into the video.)
Lost, Unwritten, & Fictive Volumes from the collection of Reid Byers
Information on the current imaginary exhibition in Paris can be found here.