What makes a book true? A moment of consideration can lead us to a conclusion that a book is never "true" in and of itself. It is the principles that are presented in the book that are true or not true. The question is further complicated by the fact that the reader may interpret the written principle differently that it was intended, causing misunderstanding regardless of the truthfulness of the intention of the writing.
To know if the principles in a book or any written text are true, one must first have a correct understanding of the meaning. Perhaps this is part of the reason why the Lord usually chooses to teach us through the Holy Spirit rather than speaking to us through a voice or vision--a heart-to-heart transmission of knowledge might be less subject to misinterpretation.