Option B is the best choice. Acid from the book end papers and glue will
migrate to the letter and destroy it, if left as it is. The removal should
be left to a professional and both letters deacidified and then stored flat
in an archival folder or sleeve and kept with the book, which has an
obvious --if not critical-- association with the letters. Do not seal the
letters in a sleeve or laminate them. Never do that to any document --the
basic rule in paper conservation is never to do anything that cannot be
easily undone without damage.
Kevin Mac Donnell
Austin TX