Book cleaner

A couple of weeks ago, while walking the dog, I found two books. They were hidden behind a pot containing plants in an alley next to my house. At first I hesitated to take them. At second glance I felt sorry for the books, because it was about to rain. I thought maybe they were left behind by a fellow bookcrosser. But no label was inside. The books already had spots of mud on them. So I rescued them. At home I tried to clean them with an old toothbrush. It took away the dirt, but it left some staining. Maybe this product can help.