Edited by Christopher Tolkien, HarperCollins

A memo to initiated and unitiated alike: this is not a spin-off.

The Lord of the Rings was a spin-off from The Hobbit (Tolkien had been asked to write "a second Hobbit": the rest is history), mined from the huge world of invented - or extrapolated - mythology Tolkien had been working on since his early twenties.

Christopher Tolkien, his son and the keeper of the flame, has now given us the unexpurgated story of Turin and Nienor, characters familiar to the serious Tolkienista from other works. It isn't jolly, but then neither is Anthony and Cleopatra.

Turin, son of Hurin, Lord of Dor-Lomin, and his sister Nienor, have been dealt a hard hand. Their father has been captured by Morgoth, the rebel "Vala", or god, whose role in Tolkien's myth is close to that of Satan in ours. Hurin dares to mock Morgoth, and so is condemned to Morgoth's curse: to see the world through Morgoth's eyes and to know his own children will be blasted by the fate which that dark power claims to direct.

But how does fate work? Is it independent of free will or is free will its agent? Steeped as he was in the saga tradition, Tolkien understood as few others have the power of tragic irony in narrative. Theoretically, pride should resist fate; in practise it fulfils it. From this paradox Tolkien derives a remarkably original tale of tragic fall. The children of Hurin do not, like the parents of Oedipus, rush to avoid their destiny: they defy it.

Pride leads Turin to ignore the help of Thingol, the elf king, not to mention his long-suffering messengers; pride leads Morwen, wife of Hurin, to a similar refusal. Pride has loyalty as its chamberlain, and even love and pity turn to disaster. There is terrible, but somehow uplifting, irony in the depiction of the Battle of Unnumbered Tears: Fingon calls out joyfully, "The day has come!" before the battle, and is echoed by Hurin at its disastrous close crying in despair, "Day will come!"

Even the apparent irony that Turin effectively forgets his mother in the wanderings which began as a quest to find her is not a reproach to Tolkien's art but to the delusion that such quests can be carried by the power of passion alone.

Lovers of The Lord of the Rings will recognise familiar motifs: inanimate objects can suck in the spirit of their makers; evil speaks in seductive half truths; even the supposedly "highest" need not necessarily be the best. Elves and men are less sharply differentiated here than in The Lord of the Rings, although the former are of course wiser and more skilled. But don't look for humour here, or for that cosiness which, spliced with high seriousness, lent the more famous work its saving accessibility. This is the world of the sagas, relentless and soul-scouring.

Those who know The Silmarillion or The Unfinished Tales will find much to delight them and much to frustrate them. Beneath the (essentially cosmetic) differences in plotting lie a fundamental shift in voice and a much richer characterisation. For all its "high tone", The Children of Hurin shows an author attempting some accommodation with the cadences of modern English. Tolkien famously wrote best when he didn't try to keep up with the times.

The dated, upper middle-class chatter of Frodo's friends in The Lord of the Rings has never lost its power to embarrass, while the sonorous cadences of Theoden, particularly as he dies, never fail to move. But the speech of the protagonists in The Children of Hurin can be poignantly, even naively, modern in tone.

The figure of Brandir, whom we first met in The Silmarillion, is softened here, and that is all to the good. His seedy act of homicide in The Silmarillion did not convince, and seemed designed to exculpate Turin. Here, Tolkien ennobles it as courageous self-defence from a man not given to courage. Turin himself is even wilder than in the other versions, giving way to a madness redolent of Welsh or Irish myth. The dialogue, although often pithy and apt, and rich in unmediated emotion, often creaks with longueurs.

Much debate, or more accurately, suspicion, has gathered around the posthumous works of Tolkien. His son has been applauded as an archivist but criticised for filling in the gaps with more than scholarly enthusiasm. But this is pure Tolkien père. Here lies a curious paradox. The version of the tale of Hurin in The Silmarillion is in several respects superior to the one presented here: it is more lyrical, more economical and more moving.

Does this book deserve a place with the great myths, the high sagas, or should it be considered an interesting relic - much as Beowulf itself was before Tolkien proclaimed it literature? Or is it simply a pretentious folly? I fear it will be considered the spin-off it was never intended to be. I hope its universality and power will grant it a place in English mythology. Hold on though - that didn't exist before Tolkien. - The Independent