I make between 5-10k a month on my apps and book. (It depends on the season. Summer and Christmas are best.)
Sign up for Amazon's affiliate program and use your own affiliate links on your links to amazon.
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Also, as commented below - I posted the first of these recurring income threads almost two years ago - I'm now compiling some stats, data and insights [1] for myself so that I can learn how to boost my numbers closer to yours :)
Also, my latest book sells nowhere near that amount. How are you marketing if you don't mind me asking?
Mostly I haven't done much marketing for it outside of my blog so far. I've been toying with some other ideas for it, but I haven't really had the time to devote to it. Mostly it's just a side project I've had fun with.