While "kennen" en "weten" can both be translated to "know" there's a difference in meaning (although they're closely related). I think you could also translate the English sentence to "we weten het boek niet" but it would mean something else. If you use kennen you are talking about whether you are acquainted with the book (or its content). When using weten in this example you're talking about the knowledge which book is meant in the first place e.g. you have to buy a book for the classes you're following at university but the professor didn't tell which one you need. So you don't know the book (Je weet het boek niet).
I had some difficulty formulating my answer so I hope this makes some sense to you :) The difference between "weten" en "kennen" is a bit tricky, maybe this link or this link can help a bit further if needed!