Moldering is another word I have never used or heard of until recently. Just as my prior post on oddment, it comes from an essay submission by someone in my writers’ critique group.
Moldering?
This means to turn to dust by natural decay. To crumble to pieces, disintegrate or waste away. (A building can molder.)
It is related to words like:
decompose, decay, rot and spoil.
Exploring the use, misuse and humor of words 
Given that definition, quite a lot of my book collection seems to be moldering, sadly.