A marketing puzzle

One of the sad truths about the writing life these days is that writers spend as much time or more marketing their books than they do writing them. Or worrying about marketing and how to improve it. Gone are the days when your publisher handled marketing--all the author had to do was write, revise according to edits, and then smile and look pretty on book tours. Today we do all the work--and it's a big cause of worry. If I don't have many reviews on Amazon, how can I increase them? Is my blog attracting readers? Should I do some paid advertisings? Sponsor a giveway? Help!
I have a puzzle that is a bit different--I'm not at all worried about it but I wish I could figure out the secret so I could apply it to other titles. In the late 1970s Doubleday published a short novel of mine, Mattie, about a pioneer woman physician on the Nebraska frontier in the late nineteenth century. It was in the DoubleD series, which sold primarily to libraries and prisons, and as those books did, it sold modestly. It went out of print and was picked up by Leisure Books, which subsequently went out of business and the rights returned to me. I put it on Kindle at 99 cents, not expecting to have many sales. Within months, the book's sales ballooned--I got what I thought were large royalty checks, and they kept coming.
Today the royalty checks are a lot smaller, but that little book keeps selling and every week it gather two or three new reviews--it now has over 300, mostly 5-star. I do nothing to promote it because I'm not sure what to do. Perhaps it's the 99-cent price; I'm sure it's not the Spur Award from Western Writers of America, though I was mighty proud of that when I received it. The subject matter isn't in-your-face enough to sustain this long interest. A book of short stories, posted to Kindle at the same time, barely has any sales and maybe ten reviews.
I wish I knew the secret. I'd apply it to The Perfect Coed, which is the only other book I have control over. But it's a good dilemma to have.
Here's the opening paragraph of Mattie:
My mother was an unmarried mother, fallen woman, they called her back in Princeton, Missouri. They called her that and a lot worse names, most of which I didn’t understand at the time, thank goodness. It wasn’t just that Mama made one mistake—me—but I had a little brother, Will Henry, and neither of us had a father that we knew about. Will Henry was seven years younger than me, and you’d think I’d remember a man being around the house about that time to account for my brother’s appearance, but I didn’t. I used to wonder if Mama had somehow gotten caught in the great war just passed or if my father had fought in that war. For much of my growing-up years, Mama never told us if we had the same father or not. When either of us asked, Mama became flustered and impatient and usually just said, “I don’t want to talk about it.” There would be tears in her eyes that made me feel guilty and cruel, so I would abandon the subject