The world’s most beautiful book is also its most mysterious. The Hypnerotomachia Poliphili, published anonymously in 1499, recounts the “struggle for love in a dream” of Poliphilo, who pursues his beloved Polia through 370 pages of gorgeous woodcuts and epoch-making typography. Their story is told in a cryptic polyglot text of Tuscan, Latin, Greek, Hebrew, and Arabic, replete with arcane references and hidden meanings.

“The Hypnerotomachia is a catalogue of every possible and imaginable foil to understanding,” writes Liane LeFavre in her 2005 exploration of the text. “On every page one is confronted by words whose meaning must be deciphered, inscriptions that have to be interpreted, episodes whose conclusion is ambiguous, a hero and a heroine who embody ideas that have to be divined. Texts and images in code, symbolic images and their interpretation, are recurrent patterns in these cryptic tactics.”

The author’s enormous erudition continually interrupts his story: He fills 200 pages with architectural descriptions and another 60 with botanical lore. The book’s patron, Leonardo Crasso, wrote that it contains “so much science that one would search in vain through all the ancient books [for its meaning], as is the case for many occult things of nature.” The author, he wrote, “devised his work so that only the wise may penetrate the sanctuary.”

Why would anyone produce such a prodigious work of art and learning and then conceal his identity? No one knows for certain. A century and a half after its publication, a French reader discovered an acrostic concealed in the first letters of the book’s 39 chapters. These spell out “Poliam Frater Francescus Columnia Peramavit,” or “Brother Francesco Colonna loved Polonna immensely.” Who was Francesco Colonna? There are two candidates by that name, a Venetian friar and a Roman aristocrat. But both lived on for decades after 1499 and neither claimed to be author of this remarkable book. His identity, and that of the illustrator, remain uncertain.

Raymond Queneau’s A Hundred Thousand Billion Poems consists of 10 sonnets with the same rhyming sounds, so that their 140 lines can be combined into 10¹⁴ different poems. Here’s an interactive version.

Milorad Pavić’s 1984 “lexicon novel” Dictionary of the Khazars consists of three miniature encyclopedias that cross-reference one another. Together they document, from varying perspectives, the causes of the disappearance of the Khazar empire in the eighth century. “Each reader will put together the book for himself, as in a game of dominoes or cards, and, as with a mirror, he will get out of this dictionary as much as he puts into it, for you … cannot get more out of the truth than what you put into it.”

Julio Cortázar’s 1963 “counter-novel” Hopscotch can be read in two ways: The reader can advance through the 56 chapters in conventional order or according to an alternate order laid out by the author, which incorporates 99 “expandable chapters” supplied at the end of the book. Thus the novel “consists of many books, but two books above all.”

Georges Perec’s 1978 novel Life A User’s Manual concerns the lives of the inhabitants of a fictional Paris apartment house. Perec structured the novel by lifting off the building’s facade and mapping its rooms onto a 10×10 grid. He then placed an imaginary chess knight on a central square and worked out a tour that took the knight to every location in the building:

He used a similar technique to assign “elements” to each chapter: furniture, animals, clothes, jewels, music, books, toys, flowers, and more were salted into the building’s rooms according to the same rules. “With so much of its material predetermined,” wrote Perec biographer David Bellos, “the place of each chapter in the novel’s sequence, the place of each room described in the block of flats, and forty-two different things to say about every room — surely the book would just write itself.”

In fact Perec wrote it in 18 months. “Writing a novel is not like narrating something related directly to the real world,” he wrote. “It’s a matter of establishing a game between reader and writer.”