Good reader understands the message in the story. A stupid reader will not understand it and will read only crippled version of the original.
It's saying that a good book is not made by the author, or how good the book is written, it's saying that whether a book is a good one or not depends on the reader himself. It depends on him to make it a good book - reading it enjoyably or taking the message of the book and either making it good or bad.
Just as "Beauty is in the eye of the beholder", so does this quotation say "The value of a book lies in the interpretation of the reader." A book is a means of communication between the author and the reader. The reader may, or may not, get the message the author tried to convey.