It is tempting to blame its sales on the poor taste of readers or inadequate publicity by my publisher, but I think there is a more interesting answer, and one that applies to many other books as well: The book the author reads is not the same as the one other people read.
At the beginning of the novel, two strangers find themselves crossing a high mountain pass together. In the course of their conversation, one of them mentions a lady who is an important leader in her organization, the other mentions his sister. What he knows and she doesn't is that they are the same person. The reader only discovers that much later in the book, so unless he rereads it that particular element of the conversation is never going to reach him. The author, on the other hand, does know it, and can be at least mildly amused by the light it casts on the interaction between the two.
Harald is divided into five sections, labelled "book I," "Book II," and so on. The title of Book II is "Payment of Debts." Given how the protagonist has been treated in Book I, a reader is likely to interpret that as a sarcastic reference to getting revenge against the young and arrogant king responsible, at that point in the plot, for the current problems. In fact it is a reference to paying back Harald's perceived debts to his two closest friends, one of them—the king's father—now dead. That particular debt is going to be paid, over the course of Book II, by providing James—forcibly—with the education needed to do the job he has inherited, something that his father, for reasons implied but not explained in the text, was unable to do. The fact that my protagonist views matter that way—sees his objective as reforming the king not defeating him—casts an important light on his personality. But it is not a point that I can expect many readers to get.
I could go on at considerable length—having written a book it is always tempting to explain it, a project usually more interesting to the author than to his audience. But the basic point is simple. I know more about my world and my characters than even a perceptive reader can be expected to get from what I wrote, and much more than most readers will get. That knowledge colors my reading of the book, making it a richer and more enjoyable experience than anyone else's reading.
Which may explain why it did not sell nearly as well as, from my point of view, it deserved to.