It's not that human skin is being used for a purpose: it's that the purpose is trivial and provides no benefit above substitution.
The analogy would be if your great-great-grandfather's tibia were being used to hold a door open.
Joke aside, I see your point.
Ironically, I think I'd be okay with my skin adorning an anatomy textbook!
From what I've gathered it's a writer/poet/actor striken by grief who ponders about what happens after death, and about the notion of a soul, both historically (he seems to be speaking a bit about hinduism and greco-roman conceptions of a soul), philosophically, and theologically.
While this still doesn't justify the initial act of using someone skins, knowing the title in french makes it clear that there is a posthumous thematic, I haven't realized that it may not be obvious to non-french speakers.
That's definitely a bit more debatable, for thematic purposes as you said, but I think I (personally) would want some utility in the book (instead of a more artistic or philosophical work) to justify a skin cover.