Stored away in the rare-book library at Yale University is a late-medieval manuscript written in a cramped but punctilious script and illustrated with lively line drawings that have been painted over, at times crudely, with washes of color. These illustrations range from the fanciful (legions of heavy-headed flowers that bear no relation to any earthly variety) to the bizarre (naked and possibly pregnant women, frolicking in what look like amusement-park waterslides from the fifteenth century). With their distended bellies, stick-like arms and legs, and earnest expressions, the naked figures have a whimsical quality, though their anatomy is frankly rendered—something unusual for the period. The manuscript’s botanical drawings are no less strange: the plants appear to be chimerical, combining incompatible parts from different species, even different kingdoms. (Click on the images to expand.) Tentacled balls of roots take the forms of animals, or of human organs—in one case, sprouting two disembodied heads with vexed expressions. But perhaps the oddest thing about this book is that no one has ever read it.

That’s because the book—called the Voynich manuscript after the rare-book dealer who stumbled upon it a century ago—is written in an unknown script, with an alphabet that appears nowhere other than in its pages. The writing system is oddly beautiful, full of looping and fluid curves. A series of distinctive letters, called “gallows” for their resemblance to a hangman’s scaffold, are sometimes conjoined with other letters, or have been embellished with elaborate curlicues by a scribe. What these glyphs signify—whether they represent phonetic information or numeric values or something else—is anyone’s guess. Judging by its illustrations, the manuscript seems to be a compendium of knowledge related to the natural world, including a section about herbs, a section apparently detailing biological processes, various zodiac charts, and pages devoted to the movements of celestial bodies, such as the transit of the moon across the Pleiades. The writing flows smoothly hesitation from one letter to the next; based on the handwriting, it’s thought to be the work of at least two and as many as eight practiced scribes, and possibly required years of labor.

I first learned of the Voynich in 2010. I was completing an M.F.A. in fiction at the University of Virginia, and, anticipating dismal job prospects, had decided to try my hand at a Dan Brown–style thriller. It was to be about “Book M,” a likely apocryphal Rosicrucian encyclopedia of arcane wisdom written, in the early fifteenth century, in “magical language and letters” by Christian Rosenkreuz and the seven other founders of the order. The protagonist of the novel discovers that the Voynich, which dates from the same time, is in fact the long-vanished “Book M,” whose secrets will, if discovered … well, you know the plot. I spent a sweltering summer in Virginia trying to learn how to write a pulpy thriller (it’s a lot harder than it looks) while devoting more and more of my time to researching the Voynich. I made an electronic facsimile of the book for my iPad using high-resolution scans of the pages, and spent hours—which turned into days and weeks—flipping through the pages, captivated by the smallest details in the margins. A tiny drawing of a corpse holding its stomach next to discarded morsels of food, on the recto of Folio 66, sent me rushing to the university library to look up poisonous plants, which led me to research medieval pharmacopeias, which led to trade routes between Europe and India crossed by Arab merchants.

By the end of the summer, my novel was no nearer to completion. But a breakthrough on deciphering the Voynich seemed closer than ever.

Stored away in the rare-book library at Yale University is a late-medieval manuscript written in a cramped but punctilious script and illustrated with lively line drawings that have been painted over, at times crudely, with washes of color. These illustrations range from the fanciful (legions of heavy-headed flowers that bear no relation to any earthly variety) to the bizarre (naked and possibly pregnant women, frolicking in what look like amusement-park waterslides from the fifteenth century). With their distended bellies, stick-like arms and legs, and earnest expressions, the naked figures have a whimsical quality, though their anatomy is frankly rendered—something unusual for the period. The manuscript’s botanical drawings are no less strange: the plants appear to be chimerical, combining incompatible parts from different species, even different kingdoms. (Click on the images to expand.) Tentacled balls of roots take the forms of animals, or of human organs—in one case, sprouting two disembodied heads with vexed expressions. But perhaps the oddest thing about this book is that no one has ever read it.

That’s because the book—called the Voynich manuscript after the rare-book dealer who stumbled upon it a century ago—is written in an unknown script, with an alphabet that appears nowhere other than in its pages. The writing system is oddly beautiful, full of looping and fluid curves. A series of distinctive letters, called “gallows” for their resemblance to a hangman’s scaffold, are sometimes conjoined with other letters, or have been embellished with elaborate curlicues by a scribe. What these glyphs signify—whether they represent phonetic information or numeric values or something else—is anyone’s guess. Judging by its illustrations, the manuscript seems to be a compendium of knowledge related to the natural world, including a section about herbs, a section apparently detailing biological processes, various zodiac charts, and pages devoted to the movements of celestial bodies, such as the transit of the moon across the Pleiades. The writing flows smoothly hesitation from one letter to the next; based on the handwriting, it’s thought to be the work of at least two and as many as eight practiced scribes, and possibly required years of labor.

I first learned of the Voynich in 2010. I was completing an M.F.A. in fiction at the University of Virginia, and, anticipating dismal job prospects, had decided to try my hand at a Dan Brown–style thriller. It was to be about “Book M,” a likely apocryphal Rosicrucian encyclopedia of arcane wisdom written, in the early fifteenth century, in “magical language and letters” by Christian Rosenkreuz and the seven other founders of the order. The protagonist of the novel discovers that the Voynich, which dates from the same time, is in fact the long-vanished “Book M,” whose secrets will, if discovered … well, you know the plot. I spent a sweltering summer in Virginia trying to learn how to write a pulpy thriller (it’s a lot harder than it looks) while devoting more and more of my time to researching the Voynich. I made an electronic facsimile of the book for my iPad using high-resolution scans of the pages, and spent hours—which turned into days and weeks—flipping through the pages, captivated by the smallest details in the margins. A tiny drawing of a corpse holding its stomach next to discarded morsels of food, on the recto of Folio 66, sent me rushing to the university library to look up poisonous plants, which led me to research medieval pharmacopeias, which led to trade routes between Europe and India crossed by Arab merchants.

By the end of the summer, my novel was no nearer to completion. But a breakthrough on deciphering the Voynich seemed closer than ever.