Discounting multiple links by the same user is a good idea. Your seconds suggestion brings some rather complex problems, for example if a comment goes like "Code Complete is the worst book I ever read" it is certainly not an endorsement, while linking to a book in most cases is. Also a sentence like "programming perl is fun" does not necessarily refer to the book.

So this would require some form of sentiment analysis and also require book titles to be uniquely identifiable.