My dad read this book in England in the late 40s or early 50s. It’s about a boy who is sick in the summer and he’s hot in bed (possibly in an attic bedroom). His grandmother keeps piling blankets on to him despite his being too hot. He eventually falls asleep, but when he wakes up, he’s not in his bed. He’s in a world that’s covered in a mattress (i.e. the ground is a mattress). He stabs a knife into the ground and feathers come streaming out. He has adventures in this magical place. My dad is the one who was delivering plot points, and that’s all he can remember about the book. Publication will not be later than the late 1950s.