What is a hypertext novel?

I was reading in "Writing Machines" about Phillip's art book that was created from Mallock's novel. In one part she writes (I'm looking for the quote now but I can't seem to find it) that Phillips said that if the novel sold well he could do 12 new pages and substitute them in the book so that the nth edition might not include any pieces from the original. "Rewriting" (through blocking and image creating) Mallock's novel has infinite possibilities and Hayle's suggests that this makes it a "hypertext." But it is only a hypertext for Phillips, not for the reader, who basically reads the artist book front to back, left to right, following the rivers. Is hypertext defined as a writer's process, a reader's process or both?