Joanne Greenberg -- good heavens. The novel (by "Hannah Green") was a serious fave of mine when I was a teenager; my copy is within reach as I type these words. I didn't know that a more detailed account of the original psychosis was available.

You might find it entertaining to read Jorge Luis Borges' story "Tlön, Uqbar, Orbis Tertius," which deals with exactly the question of what happens when a fictional world starts impinging on what we like to call the "real world"...