I would add that if a professional conservator is not available or not
affordable, leave the book and letters alone, but put a piece of acid-free
paper inside the cover (between the letter and the rest of the book) so
there is a place for the acid to migrate.  You should also carefully
photocopy or transcribe or scan or photograph the letter so the original
can be left alone.

It is also much better to store the letter flat in an archival folder (acid
free) even if you can't afford a professional to deacidify it.

Also, be sure to only use pencil to label the archival folder, include a
note, or number the document or book.

Good Luck,

Kathy Farretta, M.A.
Riordan Mansion State Historic Park
Flagstaff, AZ 86001