Most of the users have arrived organically, but I did experiment with advertising at one point using the Social Media Facebook ad platform. It ended up costing me about $.70 per new user, and I thought that was too much to spend considering the fact that I haven't really made any money from Amazon affiliate commissions yet.
One cool thing is that I seem to have many loyal, repeat users. I see a lot of the same people adding and managing their books from week to week. I also have users from all around the world including Saudi Arabia and Turkey.
Even if the app fails to expand exponentially, I still enjoy maintaining it and reading the positive things that users say like this message from someone named Anthony:
...Ken, it's an excellent app. The most useful I've found yet, well done.