letter, missive - a written message addressed to a person or organization; "mailed an indignant letter to the editor"

annotation, notation, note - a comment or instruction (usually added); "his notes were appended at the end of the article"; "he added a short notation to the address on the envelope"

matter - written works (especially in books or magazines); "he always took some reading matter with him on the plane"

back matter, end matter - written matter following the main text of a book

appendix - supplementary material that is collected and appended at the back of a book

continuation, sequel - a part added to a book or play that continues and extends it