What I learned is there was not. I got the idea for it one day and four days later I released the first version. One reason for doing it, despite an unknown but probably small market, was I could do it so quickly (I used an free open source Java library to handle the database). The success for that app was a very small one, it paid the $25 fee for me joining as a Google Play (then Android Market) programmer. The app which allowed people to read books was a larger market and was the real success.