My work on him focussed on his 1930s/40s work - bitter, darkly humorous novels. But GT's appeal is obvious: hugely intelligent, open, astonishingly fluid linguistically. This is perhaps part of the postcolonial conditions: surrounded by Welsh but unable to speak it himself (he often expressed his distrust of the Welsh-language movement), he latched onto the subversive and poetic possibilities of English in a way very few English authors do ('words are indeed weapons', he says in this interview, and they explore the idea in the second part). See also those other Welshmen Dylan Thomas and Iain Sinclair.
This verbal facility is what attracted audiences, though it's also a function of English distrust of the Welsh as facile, and divided between surface charm and a darker interior. Actually, I don't know if this kind of feeling still exists, but it's all over 20th-century literature. Not that anti-Welsh hatred has disappeared: the Daily Mail called the language an "appalling and moribund monkey language" and goes on to suggest that
Not many people in full possession of their faculties would find it appealing or necessary to try to turn themselves into a ‘real Welshman’.
which sounds like the worst kind of racism to me, and the article is a revolting mishmash of half-truths and specious generalisations.
I have to admit that as GT became more of a media personality, the weird energy of his books decreased: from savage outsider's satire, they became rather too cosy performances of the comic Welshman. Read All Things Betray Thee, Sorrow For Thy Sons and The Dark Philosophers.