The Dresden Codex, so named after the city of Dresden in which it's kept and was first recognised for what it was, is one of three known surviving books of the Maya. It is the most complete and well-preserved of the three. The others are the Madrid Codex and the Paris Codex. A fourth, fragmentary work called the Grolier Codex exists and is kept in Mexico City. Only the Dresden Codex is accessible to the public.
The book is 74 pages long. 70 of these pages are inscribed on both sides. It is made of lime-pasted tree bark folded in accordion style. It's been dated to the first half of the 13th century CE but the information in it was compiled as early as the first century BCE. The distinct styles of eight different scribes have been identified. Primary colours used are red, black and blue. Most likely it came from the famous temple-observatory of Chichén Itzá.
The Codex probably arrived in Europe as part of the royal fifth of the treasures of Mexico that Hernán Cortés sent to Charles V in 1519. From Spain it found its way to Vienna where it was acquired from a private collector by the director of Saxony's Royal Library in 1744. The Codex sat in the Dresden library for fifty years before someone made a connection between it and a similar document that surfaced in Leipzig. Still nobody was sure what it was. Alexander von Humboldt copied several pages of it in a book about the ancient people of the Americas and a few years later it was published in its entirety, though at the time it was identified as Aztec. Not until 1829 was it recognised as a Mayan document. Work on deciphering didn't begin in earnest until the 1880s though when another director of the library, the inspired Ernst Förstemann, managed to make headway with the Mayan script and numerals.
During the catastrophic allied air raids on Dresden in 1945 a number of pages were damaged, the RAF almost succeeding where the notorious book-burning bishop Diego de Landa had failed. While the Codex was stored in a bombproof underground bunker, it nonetheless sustained serious water damage from river water, water used in fighting the fires and general dampness. When it was put back together the order of the pages was somewhat confused and the late-19th century copies may or may not be sufficient to arrange them in their proper order.
So, what's in it? Beyond doubt the most comprehensive source of Mayan astronomical knowledge and the complex calendar system they used. In part it is an almanac with a smattering of cosmogony and/or eschatology, covering subjects like health and agriculture, mostly in a manner we'd call superstitious. The best part of it is a detailed calendar based on hardcore astronomy. It still hasn't been fully deciphered, though the astronomical tables have been interpreted and satisfactorily matched against European observations. Ironically the notes of de Landa, the man who strove to wipe the "idolatrous" books from the face of the earth, were key to its translation.
One of the significant realisations during the interpretation of the Dresden Codex was the fact that the Maya used a vigesimal (base 20) arithmetic system. Their arcane calendar has been studied intensively from this book and a number of Mayan gods were identified. In conjunction with archaeological finds in the Yucatan peninsula and elsewhere it also revealed the extent of the Mayan obsession with time and their remarkable precision in counting it, as well as their skill in predicting eclipses using tools and calculations that remain unknown. Whatever they were, with such advanced mathematics it was clear that the number 0 was not only known to them but vital to their work, long before it was used in Europe. The numerous tables for predicting events and scheduling religious ceremonies are not only solar but also based on the cycles of the Moon, Venus and several stars, as has been deduced from the precise dates given.
Maybe there are more revelations waiting in the untranslated text. Maybe there are more answers in the Fifth Codex. Fact is, the Dresden Codex and the lost knowledge about the Maya that's been recovered using it has not only given historians and scientists great insights into the Mayan world but has also inspired many myths and made the book a favourite reference for eschatologists of many colours and hues.
The Foundation for the Advancement of Mesoamerican Studies has images of all four codices posted on-line in PDF format at http://www.famsi.org/mayawriting/codices/. (Thanks to gaw for this tidbit.) See for yourself and make your own dire predictions about the fall of civilisation and the end of the world as we know it. Or at least the end of the world as the ancient Maya knew it.