I have experience of published work in both fiction and non-fiction on Amazon for Kindle and I can tell you that there is not a lot of money to be had from it, from my experiences. With that said, I just put them on there a left them to make money themselves, I didn't really promote them or boost them in any way, so I would recommend that you do everything you can to get more sales. I will, of course, share your links to your books wherever I can, to help you out.
Anyway, if you want to chat some more, send me a message and we can talk further.