You mean unbind it and rebind it or just stripping the cover off and putting a different book inside?
You can just cut the cover off and then glue some bound leaves in the there if it's roughly the same size. Just take a quick look at a hardback and you see how it goes in there. Other than that, proper application of watered down white glue with a paintbrush and use of a couple clamps to make sure it sets right I guess.
That's how I've rebound my old Btech sourcebooks from the 80's with loose pages and covers anyway. After 3 years they're still in really good shape.
Not read a lot of Lovecraft so I don't know the specifics of how the thing is supposed to go together. Can be a little more detailed if you provide some more details.
I mean my textbooks, hardbound novels, and my family bible from the 1800's are all hardback but bound in different ways.