Before the 15th century there was no printing. All books, letters etc were hand written and copies were copied out by hand. The most popular books might only exist in a few hundred, or thousand copies. Most books less than 100.

Books got worn out, lost in fires, sometimes the vellum was scraped down and reused. Paper copies might have a life of 2 or 300 years tops unless unusually well stored. Eventually old books were forgotten and disappeared unless someone took the trouble to recopy them.

Sometimes 'lost' books turn up in old libraries, for example in monasteries where they have lain for hundreds of years. In Egypt archaeological digs in old dumps turn up papers that were thrown away hundreds of years ago but such perfect conditions are very rare.

I mentioned how old books were scraped down and the vellum or parchment recycled into new books. Sometimes the old writing can be brought out by chemicals, infrared light etc and the lost writing recovered. Sometimes old papers were reused in binding books and these old scraps can be recovered.

Odd bits and pieces turn up from time to time. To find a complete lost book would be a once in a lifetime find.

There are some books that were burned up and buried in the Pompeii volcanic eruption in 79 AD. Experts are working on the carbonized remains and may be able to recover the texts in time.

You never know what might turn up next.