Split down the middle, the reviews there are of this book veer between "this is a shameless feminist manifesto - a betrayal of fantasy!" and "this is a fitting, moving development of the gripping trilogy". I would like to explain why I favour the second point of view.
Studying a great poet like Dante Alighieri has made me realise that the seeds of genius are always present in the work of a great artist, but they take time to mature. Dante's masterwork, the Divine Comedy, was the product of years of undeveloped thought -on love, philosophy and politics. It was only by the time he wrote and later revised the Divine Comedy, that these thoughts could finally crystallise, around an epic yet utterly humane vision. In many ways Dante had renounced some of his earlier beliefs, but this did not make all his works jarring, or inconsistent, it just showed that he had come to master himself and his beliefs.
The original Earthsea trilogy was an engaging enterprise that nonetheless set itself largely within a tradition of "fantasy". 'The Wizard of Earthsea' was a typical 'bildungsroman' i.e. the story of a young man on a quest. But within this set of conventions - the pride, the error, the journey, the temptress, the old master - lurked something deeper. Le Guin posited ideas of how one really understands the nature of power, by using the allegory of magic. To be honest, whether this was 'Taoist' or not, as has been alleged, is immaterial to me.
It is immaterial because Le Guin then went on to forge her own philosophy, which was and continues to be compelling. Tenar in 'The Tombs of Atuan' faces her own challenge; a different yet parallel fate to Ged's awaits her. In 'The Farthest Shore' Ged has his own apprentice to educate, yet the mission and conclusion are different: a King is sought to rule, not a Mage, and magic is ultimately subordinated to this new hope.
It was an end, of sorts. But clearly Le Guin then felt it necessary to understand, as a person herself, what she felt over 10 years later about her creations, and what she thought still needed to be said about the radical conclusions she had already drawn within the conventions of Earthsea. In subjecting the trilogy to a new, more objective analysis in 'Tehanu', I do not believe that she has broken the 'rules' of fantasy. Instead within her Earthsea - still a world of power, of men and women, of great deeds set against the fragility of the human spirit - we now learn of loss and acceptance, of the power of love set against that of magic, and of the fate of men and women. It is a powerful message which, while it does reflect a change in Le Guin's perception of her old trilogy, is nonetheless a change that I think should take her audience along with her. To understand 'Tehanu' is not to understand it as a 4th part of the trilogy, but as the inspiring result of a critical self-examination, and one that ultimately makes the whole Earthsea output all the more rich and intruiging.