I have read the book referred to (few years ago) and was under the impression a "coffin" quilt was one made by the family from clothes of the "passed on" member of the family, and given to members of the direct family (parents, children, sibblings) in rememance of the departed loved one.
I guess it might make a perfect October quilt project for those doing genealogical studies of familly histories with complete info on cemteries, dates of birth and death. at one time, I had 2600 names on record in my computer, but the computer crashed. May be it was too many coffins in storage of my family records.