It has already been followed by a second, The Sianian Wolf, and the other two contracted books will be published in quick succession, on October 20 and next month.
That won't be the end of the story for Ms Willemse's leading character Rafen, a teenage boy put to work in a coal-mine but later groomed to be the personal slave of the despotic king of her invented world of Tarhia.
For she has already written a fifth novel, and is shooting for a seven-book series, which she hopes to complete before she turns 25.
With a pastor father, and a mother who home-schooled her, she took her main inspiration for the series from a children's picture-book version of John Bunyan's 17th-century Christian allegory The Pilgrim's Progress.
Although she is not looking forward to farewelling Rafen when the time comes, having developed him as a character since she started scribbling in notebooks as a 6-year-old, she has a notion to follow the series with "a really humorous little trilogy" aimed at children.
"I wrote a couple of drafts for it when I was 13, so I'm thinking about maybe coming back to that."
Ms Willemse balances her writing with work as a piano and singing teacher, and is not sure whether she will be able to become a full-time author. But she appears to have the publishing world at her feet for now.