Good question.
It would be a bit easier if we could all hear as clearly as Moses could hear Him...and Abraham too.
He didn't use a book then...neither with Adam.
Personally it seems to me as if the book is like handing us a 'life test' the questions and answers are in it's pages...much like a map.
I do like your question though.
I guess the book is much like a love letter from Father to us.
( as we know, other people choose to see an angry vengeful God in the pages, that is ready to punish. )
I see Him showing us His plan from Gen - Revelation.
And the best thing of all.
God Win's ....