In the middle ages books were pressed on paper, which was made of old clothes.

This paper existed therefore from flaxfibre and was nearly indefinitely good. Bank notes of this material are still made also today. Everyone knows, tales of bank notes, which arrive in the laundrymachine and which still are in good state after washing and drying 

After the napoleonistic time more and more people learned to read and write due to better education. This resulted in the production of books becoming larger and in the middle of the nineteenth century there no longer was sufficient paper for all new books.

So people searched for other fiberrich materials. Wood appeared to be the most common and easiest to treat. since that time most of the paper was made from wood.

Paper made from woodfiber has as disadvantage that it has a grey color and contains lignine and other substances.
To bleach it to a pure white color, chemical methodes were used , for which substances  like natriumsulfite were added and washed out later on.
Often remains of these chemicals were left in the paper.

All of these substances are damaging the paper fibres, especially when it becomes wet. In general this is called  acidification.

There appear brown spots which become darker at the time. On these places the paper becomes frail.
Finally only crumbles remain, just like when the papers burns.

In fact it is a form of oxidation, only much slower. That is why it  took decades before somebody noticed  what was causing the problem.
Nowadays there are several ways to make better paper. like bleaching with chlorinegas (better for the paper worse for natur)
Also the use of wood free paper, this is infact a bit of a deceiving name, it is made of wood pulp, which is boiled with natronloog or other chemicals untill all lignine is desolved and only cellulose  remains. For making wood free paper there is more wood necesary as for making normal paper.
Almost all the paper made between 1850 and 1950, is in danger of acidification. This means that books printed before 1850 often are in good condition, but books from between 1850 and 1950 sooner or later will desintegrate, unless special precautions are taken.

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