The paperback point is right and there is a second reason it works that took me a while to notice: it is visible to other people.
A phone in a queue reads as unavailable. A book, oddly, does not - people still talk to you, and you can look up without it feeling like you were interrupted. I did not expect a social difference and it turned out to be the thing that made it stick, because the phone version of the same ten minutes was quietly making me less present in public generally.
The practical failure mode is that the book has to be in the bag, and there is a week or two where it is not because you forgot. I solved that with a genuinely stupid rule: the book lives in the bag and is never taken out at home. If I want to read at home I use a different copy or a different book. Sounds excessive; it removed the only real failure point.