Hmm... I detect that the MC's intelligence/wisdom hasn't enabled them to predict that if they take a writing implement (pencil anyone?) and write in the World Book they may be able to alter the rules of that world. For example, if the MC rewrites his 1 HP to infinity, then will the book change along with the world? Will the letters change and the world radically alter?
As a book that simply gives him all knowledge of their world, it makes sense, but on the other hand, does it give him the power to actually affect change in his world? Either way, it's probably the best gift of the 5, but I'm still curious as to what he'll do moving forward and if he'll make attempts to change the book.
If he can, then he need only change his sister's stats so he doesn't have to worry about her. >_<