It appears the book was poorly printed. Perhaps the sons of Chuck Scribner were having a bad day. If the signature is on the tissue guarding the author's portrait you can be fairly sure it's printed there. An author generally signs the title page - the issue would probably rip if you put a fountain pen to it.
A previous owner took the spine from other books in the set and glued them into this one. Why? That's a good question that I can't answer. It certainly wouldn't add any value to that copy.
Unlike coins and stamps, book errors are generally not collectible (although there is a famous edition of the Bible where they left "not" out of the Seventh Commandment; For some reason that's kind of popular).
A dealer would probably carefully cut the thread binding the page signatures together and sell off the author's portrait and any other illustrations in it.
Sorry - it appears to be a series of serious errors by the printer and for some reason that copy is still around.
Good luck to all,
The Old Bookaroo