It's not comprehensive, obviously. As Bob Dylan put it, Open up yer eyes an' ears an' yer influenced an' there's nothing you can do about it. I've probably forgotten some of the authors who had some specific effect on me, and there may be one or two I'd be reluctant to admit to, but these are the main ones. I'll go through them in (roughly) chronological order, rather than trying to work out the order I met them in.
Homer. No, not the yellow guy, the Greek epic poet. I read Homer (in translation) in my teens, as well as retellings of the stories, and I think he helped to encourage my sense of the grand and epic. Battle scenes, questing, encounters with wonders and monsters: it's all there in Homer.
Thomas Mallory. From quite a young age, I was reading retellings of the Arthurian legends that Mallory collected and framed in the way we know them today, and I later read the original. It made me passionate about everything concerning knights, real or romantic, and the episodes in the Arthurian cycle have influenced me in more ways than I can count.
William Shakespeare. Well, why wouldn't I be influenced by arguably the greatest writer ever? Everyone else is, whether they're aware of it or not. Besides his wonderful use of language, perhaps the two specific aspects of Shakespeare's writing I'd pick out as major influences are his characterisation and the sense of pacing in his scenes. I also borrowed from Henry V for a crucial scene in At An Uncertain Hour.
William Blake. Poet and artist, Blake has been described as a hero of the imagination. I think one of the big things I took from Blake is that he was possibly the first author who deliberately created a mythology with a modern(ish) sensibility. Blake's mythological figures are allegories of various aspects of the human psyche, but they also work convincingly as people in their own right.
Samuel Taylor Coleridge. Barring Shakespeare, my absolute favourite poet, and his Rime of the Ancient Mariner is my favourite poem — I could once recite it by heart, though I think too many neurons have died off since then. The Ancient Mariner is a key influence on the Traveller (who even uses an albatross as his banner) though the contrasts are as significant as the similarities. The title At An Uncertain Hour is a quote from the poem, while another Traveller story, Ancestral Voices, has its title taken from Coleridge's Kubla Khan.
William Butler Yeats. Another favourite poet, Yeats was the writer who got me fascinated in Irish mythology and the whole of the Celtic Twilight. Any wistful, magical aspect to my writing almost certainly has a flavour of Yeats in there somewhere.
William Morris. Arguably (or arguably not) the first fantasy writer to set his work in an invented world for its own sake. I first read Morris's fantasies in my late teens and was enchanted by the mixture of mediaeval exoticism and earthy reality. He was also an important socialist thinker, and a surprising amount of that creeps into his tales of knights and damsels — actually, his heroines are usually of peasant stock and far stronger than the heroes.
Lord Dunsany. Largely responsible for bringing the exotic for its own sake into fantasy, and a master of evocative naming, Dunsany created impossible palaces, the wonders of Elfland and houses built on the edge of the world. When I write of far-off, wonderful lands with resonant names, I'm almost certainly channelling Dunsany.
James Branch Cabell. Cabell, I have to admit, is something of a "take what you need and leave the rest" author. He was of his time and place (1910s and 20s in Virginia) and some of his attitudes are rather unpalatable today. If you can look beyond that, though, he was almost unique in writing fantasy that seamlessly combines comedy and romance. Perhaps his biggest influence on me, though, was the way he wove a connection between apparently unconnected books, creating a vast web of story, something I've attempted to do.
A. A. Milne. I suppose my first favourite author. I was having the Winnie the Pooh stories and poems read to me before I can remember — and I should emphasise that I'm talking about the real stories, before Disney got his hands on them. Milne was brilliant at showing characters at a stroke. One of my all-time favourite bits of characterisation is Rabbit, who never let things come to him but always went and fetched them… He's probably, more than anyone, the reason I first started writing stories.
H. P. Lovecraft. I read Lovecraft first for his Dunsany-inspired fantasy stories, but then I went on and read the Cthulhu Mythos ones too. Horror isn't my primary interest, but there are certainly dark things in many of my stories, and I learnt a lot about darkness from Lovecraft.
E. R. Eddison. Not all great influences need to be flawless. Eddison has many faults, but he was the first modern writer to create epic fantasy as we know it. The genre got diverted into elves and dark lords, but Eddison approached it politically, much as Martin does (and I do at times), and wove a Machiavellian strand into his epics.
Robert E. Howard. Now, I have been known to make fun of the stereotypical Giant Barbarian Hero (Ug the Barbarian, I call him) but Conan as originally written by Howard still stands up well. In any case, he's one of the great fantasy archetypes, and I've certainly learnt a lot about writing exciting adventure from reading Howard.
Fritz Leiber. I never read much of Leiber's SF, but his stories of Fafhrd and the Gray Mouser made a huge impression on me. There's a lot that I could pick out as influences, particularly the mixture of light hearted and adventurous, but I think what he mostly taught me was how to write a story with two equal main characters. Sometimes it seems as if the real main character in those stories isn't either Fafhrd or the Mouser, but the interaction between them. I've tried to emulate that when I'm writing similar kinds of story.
Jack Vance. Again, I've neglected Vance's reputedly excellent SF and concentrated on his hugely influential Dying Earth tales. I've picked the odd idea from his magical system for some stories, but perhaps the biggest single thing that I
stoleborrowed is his word
Overworld. Vance's Overworld and mine are radically different concepts, but
that was where the inspiration came from.
Mervyn Peake. Peake and Howard probably stand at opposite ends of some spectrum or other in fantasy, but both have influenced me in different ways. The weight and gravity of Gormenghast, its slow oppressiveness, are feelings I've occasionally tried to emulate.
C. S. Lewis. The Chronicles of Narnia were among the first fantasy works I read, along with the Arthurian legends and E. Nesbit, and they've soaked too far into me to have a hope of assessing. One influence I can't deny, though: my decision at fifteen to create an enemy in The Winter Legend who makes endless winter was almost certainly inspired by the White Witch.
J. R. R. Tolkien. I read Lord of the Rings at fifteen, and the world changed. I'm well aware of his faults now, and I don't actually want to write like him, but my ultimate ambition as a writer is to create something as great as Tolkien did. He's still my idol.
Geoffrey Trease. A children's historical fiction writer, not a very well-known author these days and has distinct faults (including his insistence on shoehorning a boy and girl into all periods, however implausible) but he got me into reading historical fiction. It makes a natural companion to fantasy, and it was reading Trease and others (certainly not school lessons) that really sparked my love of history.
Rosemary Sutcliff. Having read Trease, I got into Sutcliff, a much classier YA author of historical fiction. She wrote about a range of periods but mostly Roman Britain and mediaeval England. Among many things I learnt from her books was portraying friendship, with all its ups and downs.
Mary Renault. Sutcliff remained a favourite, but my growing love of Greece led me to possibly the best historical novelist I've read. Renault covered Greece from Theseus to Alexander the Great, and I've been told parts of At An Uncertain Hour show a strong influence from her. It wasn't deliberate, but not surprising either.
Andre Norton. Norton's Witch World series is perhaps most important for introducing a style of epic fantasy more based on people and relationships than headlong action, which has influenced many more recent authors. One very specific inspiration for me, though, was her way of writing unrelated stories set on different continents of the same world, which I've followed to an even greater extent.
Michael Moorcock. Moorcock's written so many books in so many different genres that it's difficult to narrow it down to a single influence. His Eternal Champion mega-series (which includes Elric, perhaps his best-known character) goes even further than Cabell in showing how so many different stories can be connected.
Ursula Le Guin. Earthsea is one of the key classic fantasy series, and there's a great many reasons for this. Perhaps the big influence on me was Le Guin's matter-of-fact racial mix, replacing the inevitable Eurocentric approach with a series where the main character is more like Native American. I have a whole world to play with, and I routinely have non-caucasian main characters, such as Eltava who (if she came from our world) would be half Chinese, half Native American.
Doctor Who. OK, not a single author, but Doctor Who as a whole is one of my most important influences. I watched the first episode in 1963, and I've been hooked ever since, but it wasn't till someone pointed it out a few years back that I realised how much of the Doctor there is in the Traveller: a long-lived wanderer travelling in a miraculous ship, sometimes with companions, and getting caught up in causes by his devotion to justice.
Bob Dylan. Like Tolkien and Doctor Who, Dylan has been one of the overwhelming facts in the life of my imagination. His influence is more obvious on my poetry than my fiction, but he's certainly had a lot of effect on my ways of thinking and my habits of phrasing.
Karl Edward Wagner. He doesn't get a lot of mention these days, but Wagner had an intriguing habit of writing fairly standard sword & sorcery plots in an almost expressionistic way, giving them a strange and eerie feeling. Most of all, though, it was his immortal, nomadic Kane who gave me the idea that I could have a hero wandering through history, against the backdrop of different countries and civilisations in each story. It's served me well with the Traveller.
Well, it turned out to be a longer list than I expected when I started, but these are the most important authors who formed the writer I am now. There were plenty more, of course, and a lot of authors, from Mary Shelley to James Joyce to Poul Anderson, whom I enjoyed but can't pick out any specific influences.
It carries on, too, both with classic authors I didn't read till later (Chrétien de Troyes and George MacDonald, for instance) and newer influences. I've learnt a huge amount from reading both Mary Gentle and Iain Banks, but that's a refinement of the authors listed above, who formed the unformed in me.