How many subs have a book written just for them?

The Joy of Not Working: A Book for the Retired, Unemployed, and Overworked — 21st Century Edition https://preview.redd.it/67do1ngcz5k81.jpg?width=342&format=pjpg&auto=webp&s=4f57fd98ff1bbfb2cdf7d9ba1a87b01c9a730be9