I was once telling a friend about a book I had just finished. They were curious about it and asked me to borrow it. I politely accepted knowing very well they could never read my copy for it was embarrassingly covered in the messy marks I had enjoyed making during my read. Fortunately neither of us ever brought the subject again and I now think twice before messing up my books!

I wouldn't consider those things embarrassing. You may even consider it an opportunity to discuss the book at a later date?