What is this in my book ?

It looks like mold and has been spreading but it has none of the characteristics of mold. It's not slimy or dry and powdery but it seems to be in the actual page and is black, red and yellow. It was there after borrowing it to a friend who took it on holiday and admitted that it got wet and she had not dried it out properly. I want to get rid of it as this is one of my favourite books and I don't want to destroy it. Any help would be great.