They are probably tears in the book, stains in the book, rips on the spine, writing in the book. It probably is referring to boo boos, the kid like term of getting hurt. Basically, anything that can...
I thank its the twight books
Booboo used to suck both of his thumbs and make a face so his parents called him Booboo.
Cause We All Boo Boo ;)