This is just speculation, but it's probably due to some combination of oxidation (which is the process that turns old paper yellow) and contamination with microorganisms such as mildew. Even books kept dry and contaminant free will probably pick up enough moisture and spores from the air to get a bit of mildewing.
- jpetruccelli - 13th Feb 08
I think John (bored chemist) is away at the moment. Here is an answer he previously gave to the same question. Hope he does not mind me quoting it here?
"Books, like many other things, are subject to bacterial and fungal attack- particularly if they are not kept dry. The products of this decay include volatile organic chemicals like acetic acid and these are responsible for the smell."
Similary, this from Eric:
"Actually it is not the acid that is broken down (the acid used was - and is - aluminiumsulphate, also known as "paper maker's alum) but the lignin and some of the resin that are slowly broken down by the acid.
Alum is used for fixating the resin (it is the Al-ion that performs the trick), and the resin is used to make paper more waterresistant. At one stage, alumiumsulphate was replaced by alumiumtriformiate for making "neutral" paper, but this switch is rather expensive.
The use of alum and resin became standard in around the middle of the 19th century. Before that, water resistance was obtained by using animal glue (from bones). Both the "rags and bones" that were collected were mainly for the paper industry."
This from queequeg
"In addition to mold smells of various kinds, I have noticed that some old books have distinct vanilla notes in their smell. I attribute this to breakdown of lignin to give vanillin over time. Anybody else noticed this? "
And this from Bernard (that mad man)
"I thought that it may be that smoking was very popular several years ago, having a fag while reading maybe?"
- paul.fr - 16th Feb 08