I believe that the Form of a book exists but I have to agree with Aristotle's idea that the Form is created when the object is created. I also think that Forms are eternal, but from the moment that they are first thought of and then brought into the physical world.
At one point in time someone decided to put information together in the form of words written on pages put between two bindings. Before the first person who thought of and created this "book", I believe one must ask themselves if the Form existed. How could a Form exist if the object didn't yet exist? A Form for a thing such as a book exists once the object is created because it is a new idea. Therefore, everything participates in a Form because everything tangible exists.
The idea of a perfect book can exist only in our minds through thought. Since books range in topic and in content , what might be the topic and content of this perfect book? A book contains ideas, characters, etc., so would the Form of a book then contain perfect ideas, characters, etc.?
To define what a book is is to define what the Form must be a perfect version(idea) of. My philosophy textbook differs very much from a children's book such as "Where The Wild Things Are", but they both still participate in the Form of a book, being that the Form of a book only dictates the shape and not the content. It is when the thought of a book is represented in the physical world that content and topic come into play.
Suppose that I make photocopies of every page out of "Where The Wild Things Are", and staple them together at the corner. We could still call that stapled together group of papers the book "Where The...