By Patricia McKillip.
This originally consisted of three volumes: The Riddle-Master of Hed, Heir of Sea and Fire, and Harpist in the Wind, but it was republished as a three-in-one paperback with a brief but interesting introduction by the author.
The story is complex and somewhat difficult to get into. New facts pop up like mushrooms, where you least expect them: I found myself having to refer back to chapter 1 while reading chapter 2, thinking "Wait a minute--he has stars on his forehead? How did I miss that?" Well, I don't think I did miss it. The author neglects to mention some things at first. The relationship between Morgon and Deth seems also to sprout up like magic; it seems to appear rather quickly, although the fault is mine for reading too quickly. The astute reader (or one who's read a book like Apropos of Nothing) may guess the answers to some of the many questions raised rather far before the end.
The story seems to me much darker than most of her later novels (the ones I've read), and lacking some of her beautiful charm and descriptions, although that might be fitting for the nature of the book. As her introduction says, this is the story that was closest to her childhood's heart, and certain parts of it seem somewhat unformed as a result.
The riddles referred to in the title are more along the lines of instructive catechism. "Who was such-and-such, and why did he die?" There is a proper answer to be memorized (or discovered) for each, and a moral lesson or stricture to be learned as a result, although the author leaves it to the reader to determine the lessons from most of the riddles actually unraveled in the book, rather than the past ones merely referred to. This is not a book to be read with your brain turned off. It is a plot that requires thought and careful attention to details. Some things are left implied and unexplained. Familiarity with Welsh or Celtic mythology may help with your understanding; the most obvious reference (to me) is the presence of landlaw, a mystical binding between ruler and realm, which also appears (in somewhat stronger form) in Jo Walton's The King's Peace and The King's Name (which I recommend as interesting and somehow more tangible than the usual fantasies: they are set in a very solid, well-defined world, even if being close to death might mean being a lingering spirit... the joke is funnier in the book).
Not quite as beautiful as McKillip's later works, Riddle-Master is a darker and more challenging read, with some rough edges, but it might well be worth checking out.