Also known as kormahn in D'ni language, a Descriptive Book contains the primary description of a world written according to the principles of the Art, establishing the founding link to one of the infinite Ages in the Great Tree of Possibilities.
Infinite worlds of the Great Tree can match any possible written description, and a Descriptive Book would "choose" one of the possible Ages that most closely match its descriptions. There is an obscure chaotic element in how the Book selects which of those many worlds it will link to, and the D'ni people theorized that this chaotic element was influenced by the unique physical properties of each book.
The process of describing the Age focused on avoiding written contradictions which would result in a link to an unstable Age. A Book with contradictions would attempt to link to a quantum reality that matches a contradictory description, and the closest thing it could "find" would be an unviable or unstable Age.
No two Descriptive Books could link to the same Age, nor a Descriptive Book could be duplicated (although it would have multiple Linking Books linked to it). Even if two Descriptive Books had the same content of text, they'd still link to similar, but separate, Ages, chosen per their own chaotic elements. As the chaotic elements that established a link had to do probably with the unique physical properties of each book, no two Descriptive Books could be exactly identical, thus their differences would provide different links. The D'ni made experiments to prove this theory by producing identical Books, but they were never successful.
The above means that if a Descriptive Book was damaged, the link would be lost for ever; even if the Book was rewritten, it would be unlikely that it'd link to the same previous Age, but to a similar one.
Descriptive Books have always been written in the D'ni language, believed to be appropriate to routinely handle complex descriptions. For this, no attempts have been known to write a Book in any other languages.
Alterations in Descriptive Books
The D'ni concluded that the probability waves that shape each Age never cease to exist, but each possibility continues to exist in an alternate quantum reality, until that also is observed ad infinitum. Every infinitely possible quantum combination exists somewhere in a quantum reality. Thus, even when an Age had been established and locked, there were probabilities that hadn't been locked down by description in the Book, or by physical observation in the Age itself; it was therefore possible to make changes in Descriptive Book, altering the elements that were still "unobserved".
This required meticulous attention to details and could be done by the writers of the highest level. This resulted to a synchronization between what was written in the Descriptive Book and what was observed on the Age itself. Thus Gehn was able to remove the Whiteness from Age 37 (and rendering it unstable), and Catherine caused the quakes in Riven. Atrus was also able to make corrections to the Riven Book, stabilising it. He also wrote a ship into the Stoneship age,  and wrote in the Nara domes in Haven and Spire.
If the new changes contradicted what was previously observed (thus collapsing probability waveforms), the Book would be damaged, or the link would divert to an unstable Age, or at best to a distinct (albeit a closely similar) quantum reality; whatever the case, the previous link would be lost for ever. Because of the risks inherent in altering an Age, at some later point in history, the practice was banned, once an Age had been approved by the Guild of Maintainers and the writers had to complete the Book before ever linking to an Age.
Linking vs. Descriptive books
There are several differences between Descriptive books and Linking books. The first and most noticeable of which is size. Descriptive books are much larger than Linking books. The size is a result of the necessity for the most detailed description of an Age. Linking books can be smaller and thus portable, as they are less detailed. Several copies of Linking books can be made, as opposed to the single Descriptive Book for that Age.