A piece of paper is not bad, a book or booklet is much better though, or you keep the piece of paper somewhere it can't be found or damaged, and only you know where that is in your home.
I am in full disagreement. I think storing it in a book is one of the worst things you can do because so many people do it and I think it is even one of the basic targets of a robbery.
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 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.