"written" in your sentence is the past participle and as the shortened relative clause is derived from a clause in passive voice, "written" has passive character.
In Latin a third base form is called perfect participle passive. In English this term was simplified to "past participle" As the third base form is not only used for passive verb forms but also for active verb forms (all perfect tenses) the term perfect participle passive does not fit so well. It is only appropriate when used for passive verb forms.
Your view that "book" is object to "write" and that it has become subject is totally queer. In "Harry Potter is the best book" "book" is a predicative compliment (in nominative; after the linking verb to be).