I have found the book at the putaria ruins. There is a locked door and also a room in the middle that contains the key. The door is one of the red doors that you have to force open like you do on the raids. How are you meant to do this without having other guys with you on a raid?
You should be able to ket the book of knowledge as this is separate to the room with the key. You will come back here at a later date on a quest where you will have someone with you and together ye will be able to force open this door to get the key inside. The contents will not lead you to anything special, its just story related.