The main weakness it has is from a nosey flatmate, spouse, or child in the house.

I disagree. Using this book will always lead to shorter passwords that are easier to type. That’s the main weakness imo.

Or in other words: it really depends what the user fills it with. It should be accompanied by a little machine that spits out random passwords, I’m thinking a rubics-cube-shaped bling pendant at the end of the bookmark band.