You can see some preview pages of the book here (the book itself appears to be easily obtainable as well):
https://www.amazon.com/Greatest-Albums-Youll-Never-Hear/dp/1844037770The book does indeed appear to feature all fake, newly-created album covers. I'm obviously not an expert on all the bands featured in the book, but covers for the Beatles "Get Back" and the Beach Boys "Smile" are also newly-created for the book apparently (and, no offense to the artists in question, are pretty awful-looking compared to the actual original cover art which obviously does exist for those particular albums).
As only scattered pages can be previewed for the book, I'm unclear if the book commissioned all new artwork for the albums because there were legal or financial roadblocks to showing the actual original album covers (where extant), or if it was just meant to be an artistic exercise on the part of the artists, or something else. I'm guessing not all of the albums featured in the book have extant, available orignal art. But certainly some do, and they didn't use the actual "Smile" art for instance.
I had been thinking that the "Sweet Insanity" cover seemed to be too nice and clean compared to what we probably would have gotten in 1991/92 had "SI" actually been released. I'm guessing we would have seen something more cluttered and less slick.
Still, re-creating that album art by literally "eyeing" it was pretty impressive. I'm happy to sub in this fake art rather than this: