A book is the perception of a series of events through a writers eyes. This being the case a perfect book can only be found through the eyes of a reader who can find no fault with the book they are reading. I personally have only had this experience a few times in my life and when I am reading through a book that I consider perfect there is no feeling in the world that is greater.
To me the perfect book must have a sensational story. The story line I feel is the most important part of the book. The events playing must first be understandable to the reader. This factor alone will release many potential books because the reader must either be able to perceive the events the author puts forth or must have already experienced the events within the story line. There are of course exceptions for authors who have the ability to take a story or event never perceived or experienced by a reader and bring forth the idea with such description and explanation the reader may live the experience through the writers words.
If the story is understandable it must next be interesting. Again here we lose many books because everyone has a different definition of interesting. Some simply have no interest in fiction and only find topics such as history, english, and other technical topics interesting. There is also those who only enjoy topics of specific genres interesting such as horror, fantasy, mystery, and romance. For me I thoroughly enjoy horror, mythology, philosophy, psychology, and other related topics. Again there are exceptions to this rule and stories that transcend a specific genre finding interested readers across the board.
If the book has not been eliminated by the previous two factors the next thing to look at is the characters. After reading many books I have found this is where the majority of authors struggle. This is where understanding of the human experience comes into play. The character must fit into societies conception of an individual. The reader must be able to relate and understand the character. The actions and words of the character must seem real and bring forth a dynamic personality. For a book to be good the reader must feel something for the characters he is reading. Usually the stronger the emotion the characters bring forth the better the book. The feelings do not need to be happy and in fact I have found some of the best books provoke sadness, hate, and fear.
When a book can take these three things and place them in a book with quality technical writing it has the makings of a perfect book.