You can not get the book until after you become a full dragon knight (you need to fly above the boat and transform to drop down onto the deck). Likewise, you will need to wait to finish the part of the story with the quarry tower in Broken Valley (in the original release you can get in and do part of the quest early; in DKS / D2:DC the door is locked until later). Everything else you want to do in Broken Valley or on Sentinel Island should be done before taking over the battle tower (there is a warning before that point).
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