Amazon and Book Review

I am trying to add a book review function and want users to be able to add a book and also review a book. I have been considering using hte amazon module so they just enter in the ISBN number to create a book review. Whether i use this or not my main question is how can i stop users from entering a new book when it already exists. for instance user A comes along and creates War and Peace and instead of user B just making their review a comment, they instead create a new node for War and Peace

Really appreciate some guidance