I can't seem to fathom what could be wrong in storing or writing it on one of the pages of a book, it is just the same thing as writing it on a piece of paper, the only thing is that a single piece of paper can easily be flung around and damaged quicker than a book, that is why the owner has to keep either of them well, depending on what you choose to use.
If you aren't locking it in a safe, then it is significantly easier to hide a piece of paper than it is to hide a book. A book is pretty much limited to "hiding in plain sight" - on a book shelf with other books, in a drawer, etc., and simply hope no one opens it and looks in it. A piece of paper can be hidden in more places than you can count. Take a wooden shelf off the wall, drill a small hole in the back of it, roll up your paper and place it inside, and put the shelf back on the wall. The same can be done where two pieces of wood meet in any piece of wooden furniture - chairs, sofas, desks, cabinets, etc. You could even do the same on the top or bottom of a wooden door (with a small piece of tape to make sure it doesn't fall out). Unscrew a light fitting from the roof or an electrical socket from the wall and stuff a piece of paper in there (be sure not to cause a fire hazard). Between a picture and its frame. Inside pretty much anything that can be taken apart. Under carpets or floor boards. Hell, inside your walls if you really wanted. The possibilities are endless. It is entirely possible to hide a piece of paper somewhere in your house that a robber would not find it even if he had free reign to search for an entire week. Books on the other hand are a "classic" hiding place - everyone's seen movies or TV shows where people have cut out the inside pages of a book to create a secret storage compartment.
If a robber knows you have bitcoin and targets you, i do not think any method of security can save you completely, it can only mitigate the loss, and a book doesn't make it easier for them. They can beat the sh*t out of you till you surrender some coins to them, and the amount you surrender depends on how savvy and smart you have kept your coins prior to the event, but for the sake of your life, you have to surrender something to them.
Well, that's a separate issue which is dealt with by using hidden wallets, passphrases, and plausible deniability.