Sup /an/. I've seen some very small insects residing in my old books and shelves. I've googled and they don't match the silverfish or booklice, which are the more common insects found amongst books. If magnified, I think that they would look somewhat like the insect in my picture. Any clues?
Are they also red?
Looking for a serious answer.
OP's pic is that of a dust mite. Are dust mites visible to the naked eye?
Since the photo is not that of an insect (mites are arachnids, related to spiders that is) that makes me wonder if the "insects" in your book are actually insects either. I haven't heard of mites that would live in books, though.
could be Chelifer cancroides