I have a long-term fascination with the prophet and enchanter Merlin, who I regard as representative of an archetypal figure of the human imagination.
One of the very best books about this character are the three novels by Mary Stewart, that begin with The Crystal Cave. I find Stewart's Merlin to be a very sympathetic and impressive character - such that - to have written him - the author herself must have been (or contained) similar wisdom and intelligence as her hero displays.
In this respect Stewart reminds me of Tolkien; because the 'secret' of the unmatched excellence of Tolkien's work, is the true greatness of the man himself - his depth and goodness, intelligence and learning. And it is the lack of such qualities that consigns Tolkien's many followers to a lower rank of quality - however good they may be at the craft of writing.
While not of Tolkien's unique greatness; Mary Stewart must have been a person of exceptional quality to have the ability to create scenes of great power and memorability.
Although she was regarded by many as the best of 'romantic novelists', that is a woman-orientated genre; and so I first heard of Stewart only when I was a don at University College, Durham, and studying in the English school. Some of the older academics had known of Mary Rainbow (as she then was) from when she was a brilliant student, then a lecturer, in English, at Durham; and spoke of her with a affectionate and respectful remembrance that I found striking (although I cannot recall any details).
I then thought no more of her, until the Merlin trilogy was recommended by my wife, who had read it as a teenager (and also several of her romantic fiction books - my wife has read a lot more modern fiction than I have).
I enjoyed my first read-through enough to embark on a re-read a few years later - i.e. just recently, and continuing (mostly listened-to as audible books, partly read as text).
I have some reservations about the trilogy.
It is generally slow moving, and in particular takes a while to get-going in the first book. It has no humour at all! and the seriousness, and bleakness of some setting, can be oppressive.
And the third book (The Last Enchanter) goes off the rails in the middle, by trying to stick to some medieveal/ chivalric plot elements taken from Malory; which do not fit this novel's Dark Age setting from the 400s AD.
But taken overall - and judged by its many high-points - the Merlin Trilogy is a great achievement. And the character of Merlin himself is one of the most realistically- and deeply-admirable you will encounter outside of Tolkien.