I'd been making about $30-$40 a month with Amazon for a long time, selling maybe ten or so books, and it was a decent side income without having to do anything (except initially write the books). I monitored the sales a bit for the first year or two, and it was my ACT-like Catcher in the Rye teaching guide that did the best. Then I got some poor reviews from people who apparently believed a teaching guide is supposed to include a copy of the book. Surprisingly, my family and friends who could have come to rescue me by bumping the stars up a little just ignored my pleas. That's fine, I figured, since $30 a month wasn't a big deal, anyhow.
Then I suddenly made $200 for two months in a row, and then another $150 or so, and I'm wondering if anything changed. Perhaps one of my other books got more popular, or people just blew off the low star-rating on my best seller. My wife wanted me to check on it, but I figure as soon as I check on anything, I'll ruin the momentum. Actually, it's gone down each month since the initial influx, so the momentum is probably lost. Maybe some teacher recommended it in a forum post that is getting fewer and fewer hits. Or it was a different book entirely. It doesn't really matter, since I'm not going to change anything.
Oh, and advice for potential Amazon authors? Write several books and get them on there--you can't sell them if they are not on the site. Promote them, but not just with family and friends, since those people probably think you suck at writing, anyhow. Hope for the best. Don't be surprised if the book that sells the most isn't the one you thought would sell the most, and don't assume that adding something very similar will also sell nearly as much. There's probably some kind of algorithm out there that decides all this. Like the algorithm that got you to this article.