Carpet Beetles

Adult carpet beetles feed on pollen and nectar and as such aren’t a threat to book collections. The larval stage of carpet beetles however are more attracted to cloth covers of books and can be very damaging as they tend to eat a lot. Carpet beetles thrive in environments like dirty carpets where fragments of hair, skin, pet fur and insect remains and provide a ready source of food.

The life cycle of a carpet beetle.
Source; http://iowabridalpreservation.com/tag/liscenced-pest-managment-professionsl/

‘Frass’ – insect excrement is one of the first signs of infestation. It is often pushed out of the larva burrowed tunnels when the adult beetles emerge.
Source; http://britishlibrary.typepad.co.uk/collectioncare/2013/08/the-bookie-monster-attack-of-the-creepy-crawlies.html