It depends a bit on the materials used.

For example, papyrus, made from reed usually detoriates to the point of being unusuable in about 20 years in any kind of damp climate (such as the European one), while parchment, made from animal skin handled damp a bit better (although it was prone to buckling with changing humidity).

The Dead Sea scrolls were very well preserved by the dry donditions they were buried in. They were made both from parchment and papyrus.

Likewise, there are Egyptian letters that were buried with refuse in a pile at a tomb construction project that surives to this day.

In northern Europe, the often oxygen-free laters of waterlogged clay found in many places can sometimes preserve organic materials very well, as seen by various bog bodies and other finds. Birch bark were used to scribble on, and the works of a young boy called Onfim found close to Novgorod is famous. He was doing home work, scribbling knights and drawings of himself and his teacher in teh 13th century.

Early modern paper, made from linen, hemp or cotton rags is pretty stable and lasts a long time, especially if dry. More modern wood pulp paper, especially the cheaper kind is acidic and darkens with time, especially if exposed to sunlight, and turn brittle and might even crumble with time - old news paper clippings are good examples of this.

Many things depend on when, how and from what your hypothetical book was made. If a modern book of loose sheets bound together by glue, the binding might dry out and crack when someone tries to open the book. A scroll might become brittle and dissolve when someone tries to unroll it.

An ideal storage is most likely cool, dark, dry, unaffected by pollution and the book is made from high quality hemp, cotton or linen paper.

Since books and scrolls were historically made of organic materials, the same conditions that preserve dead bodies are also generally good for preserving books and other written material. That's why the dry desert conditions that helped preserve dead bodies in the form of Egypt's mummies, also worked to preserve the Dead Sea Scrolls.

The major enemies to book preservation are insects and molds. Bookworms are actual worms that burrow into books and make holes and will eventually eat up the whole book. Mold grows on books and are far more likely to grow on them if conditions are damp. Mold is contagious, too, and spreads from book to book. I've seen 100 year old books that are covered in mold and filled with wormholes, and had been stored in tropical places. Or that had been stored in New England, where it is damp and moldy. Books stored in the desert are in much better shape.

Another important consideration is the quality of the paper. Paper before around I think 1820 was high quality and expensive, so there were a lot fewer books, but the paper lasted longer. You can see paper from 1800 that is creamy and white and strong. After 1820, there were a variety of paper qualities, including cheap paper, like the paper used to make phone books and newspaper today. That paper crumbles to pieces after a few decades. Paper of slightly higher quality, like the kind used to make most hardback books, won't crumble like that, but it's acidic, as is the ink. Pages darken and burn shapes in the pattern of the words into adjacent pages due to the acid. It can make it hard to read, depending on how old the book is.

So you'll want to have things stored in a cool, dry place. Very very cold is good--they're inherently dry. You want to have high quality paper as well.

You should read more about the Dead Sea Scrolls, since they were basically put in a dry cave, sealed off, and just left there for two millennia. They were preserved very very well, all things considered, but at least some did break up into pieces that had to be painstakingly put back together.

And you can see paper from 1950 that is falling