My sister asked for a small go bag that would fit a single book with room for the typical things (cards, phone, mints). I’ve browsed some options(including bags that incorporate a book strap) but wanted to check what other people who carry books around typically prefer before committing to something.
I use the Fjällräven/Specialized (now seems to be rebranded as ‘Hoja’) hip pack for this.
It’s just about big enough to fit an average book, my headphones, and some miscellaneous stuff. But if needed it’s also expandable so you can fit a jacket, or groceries on the way back etc.
It has completely replaced the backpack I used to carry everywhere (which I carried because of wanting to bring a book).