It is a fine morning in the Holy Roman Empire. The year: 1431. The tradespeople line the banks of the great river. The bookseller is there, his timeless green box crowded with books. My medieval eye spots a limp vellum item, what you out there would call a paperback. Still, the parchment is good stuff — this skin of a calf has been scraped to whiteness on both flesh and hair sides — and illustrated on almost every page. True, the artwork is provincial, and the color washes — blues, reds, greens — are clearly cheap, though phantasmagorical and full of brio. The written hand is now crabbed, now undulant (more than one scribe, then), and the presentment of the illustrations with the text — no overlaps, no bleeding — is orchestral. The language, however, eludes me. Nice and quirky, but I want something to read, and move on.

Well, I just passed on what would later become known as the Voynich Manuscript. A more enterprising someone, 150 years later, would sell the indecipherable whatever it is to Emperor Rudolf II, for 600 gold ducats, because the emperor enjoyed a good mystery, secret writing and alchemy. Who knows? The book could be a communication with our angelic hosts, or a fabulous work of alchemy.

All this and more is laid out in "The Voynich Manuscript," a mouthwatering facsimile of the incunabulum, complete with foldout pages — one origamied into six folios — and a handful of scholarly interrogations.

It is a crack, affordable reproduction of an uncanny nonesuch: a round of applause to Yale University Press and the book's editor, Raymond Clemens, curator at Yale's Beinecke Library.

What we have here is a book in an unknown language, filled with illustrations of plants and star maps unfamiliar on Earth: no title, no known author, unreadable. Not a word about its social circle, its cultural surrounds.

The six essays that accompany the volume are absorbing squibs, as frank as Sgt. Joe Friday: "Just the facts, ma'am." Two chapters introduce the book itself: its parchment, ink and pigments; its rebinding; the removal of folio pages (all unrecovered). Gratifyingly, these essays detour into parchment making — "disgusting" — and the source of pigments: oak gall, iron, azurite, copper, egg white, ocher.

Two more chapters discuss the manuscript's spotty province and travels, from Prague to Hamburg, to Jesuit libraries and, finally, in 1920, into the hands of antiquarian book dealer Wilfrid Voynich, a "lovable rogue," with "an undertow of deviousness" — through whose hands it landed at Yale.

The cryptologists have made little headway, which doesn't make their conjectures less engaging, from the Roger Bacon episode (parchment not old enough) to microscopic notation ("individual pen stroke within a single character, when magnified serve as shorthand symbols for other letters"), with plenty of academic beard pulling in between. Philologists are befuddled, but aren't of the mind the manuscript is a hoax: The lettering has too many known textual exemplars and stylistic conventions. "None the wiser" is the refrain.

So far, alluringly, the manuscript is keeping mum. But listen: An applied linguistician recently claimed to have deciphered the words "Taurus" and "centaury," an herb. Also recently, the American Botanical Council published a paper suggesting one of its plant drawings intimates a Mexican connection. The Voynich likes nothing better than deepening its mystery.

Peter Lewis is the book review editor of the Geographical Review.

The Voynich Manuscript
Edited by: Raymond Clemens.
Publisher: Yale University Press, 304 pages, 268 color illustrations, $36.99.