Yeah, I find hard to finish things when I'm dead too. It was actually finished off by his son, wasn't it? Now I'm going to have to go and check...
...Doesn't say, and the way my brain has been working, I'm probably thinking of something else anyway. But here's something else interesting from the introduction:
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Among his other writings were "Shapes and Sounds" (1941) and several volumes of exceptionally accomplished poetry; zany nonsense verse and stories such as "Rhymes Without Reason" (1944) and "Letters from a lost Uncle" (1948) and "Mr Pye" (1953), a novel with allegorical overtones set on Peake's sometime island home of Sark. He also wrote a number of plays including "The Wit to Woo" (1957), the critical failure of which induced a nervous breakdown.
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So he did write other stuff, but it may not have been fantasy.