I increasingly distrust the terms "science fiction" and "fantasy" because so many people, particularly those who rarely, if ever, read anything in either genre, tend to see them as being narrow and rigid categories, as a way of excluding books from consideration rather than bringing them into the fold. The truth, of course, is that there are untold numbers of writers out there (including many literary and even Nobel-winning authors) whose works flirt on the edges of SF and/or fantasy or flit between the two and untold more writers whose writings, while fitting into neither genre, have the same spirit of adventure and enquiry that informs the best work in both. As a result, I've started to prefer the term "fantastical writing" which seems to cover much of the non-realist fiction I like best.
So, what exactly do I mean by "fantastical writing"? As it turns out, it's extremely hard to define. For me it's writing that succeeds in combining a sense of the unheimlich (somehow a much more precise word than eerie or uncanny) and of the wondrous, the sort of writing that can leave one at once both open-jawed at an idea whilst at same time feeling a shiver down one's spine. It's writing that is informed by the spirit of fairy tale, taking us to a place that, wherever it might be set, runs according to rules slightly different from our own. It's also writing which, at the top of its game, manages to amaze and unsettle even as it echoes to the sound of gentle laughter. It's writing that... No, let's face it, this really isn't getting me very far at all. Instead of continuing my attempt to ham-fistedly flail a definition together, why don't I just list some authors I think fit the "fantastical category" nicely and leave you to work out the definition from there? It's in no way an exclusive list, in general I've tried to look at authors more firmly lodged in the "literary" than the more general "fantasy"/"sci-fi" camps, partly because I'm dreadfully pretentious but also because wonderful writers like Ursula Le Guin, Neil Gaiman, Margaret Atwood, Michael Moorcock et al appear on plenty of lists already.
So, with the preamble out of the way, let's get on with the first part of the list.
Čapek is a slightly odd inclusion in this list, not because he lacked any sense of wonder but because, far from his writing being unheimlich, he somehow managed to make the most extraordinary ideas - the enslavement of a newly-discovered race of humanoid newts, a scientist's new power source creating little bits of the essence of god, a robot uprising, a man who meets multiple aspects of his own self - seem absolutely real, playing them out with total conviction amid a cast of characters at once as comical and as true to life as you could wish to find. His books are suffused with a deep understanding both of mankind's follies and of its potential. His satires in particular, while as clear-sighted and unsparing of human foibles as anything in Swift, are filled with a sense of gentle laughter. In the 1920s, Capek was one of the world's most famous writers, his books translated into multiple languages, plays like RUR and The Insect Play appearing in productions on Broadway (Spencer Tracy played one of the robots in the former) and London's West End and The Makropulos Affair going on to form the basis of Janacek's celebrated opera. Now he is best known for having invented the word "robot", a term in fact coined by his brother Josef. Do your best to help put that right: go away and read War with The Newts or The Absolute at Large.
Jorge Luis Borges
[For Part II of this list, see here]