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.
Alpaka Go Sling Mini might work for you.
I made this post a while back on R.
https://www.reddit.com/r/ManyBaggers/comments/1hodsuw/i_usually_dont_carry_all_of_this_but_its_nice/
There is a comment link I made that shows several books sizes and how they fit. There’s also a ruler so you know the dimensions of the books.
They have a larger size also.
Edit: Here’s the book sizing.
https://imgur.com/a/what-book-size-fits-alpaka-go-sling-mini-v2-O8z08V7
There’s also this but it looks smaller.
https://www.fjallraven.com/us/en-us/bags-gear/backpacks-bags/daypacks/high-coast-crossbody/