I had never heard of this strange little aside to the Gormanghast trilogy but, among Peake fans, this is considered a rare gem of a read.
Nestled somewhere inbetween Titus Groan and Gormanghast, this novella tells the story of Titus on his 14th birthday. In a fit of Pique, he leaves Gormanghast, finding himself in a desolate gray, dusty land populated by two strange beast-men and a freaky little lamb. It's quite weird and very short but has moments of brilliant prose. If you can find a copy, I recommend it.