雁書 is read as がんしょ (gansho) and means letter.
がんしょ
gansho
かり
wild goose
ぶんじょ
document addressed to someone
ずしょ
books
とくしょ
reading (a book)
かたがき
title (e.g. Doctor, Professor, Lord)
しょうしょ
document (of proof)
ひしょ
(private) secretary
ちょしょ
(written) work
きょうかしょ
textbook
とうしょ
contribution (to a newspaper, magazine, etc.)
りょうしゅうしょ
formal receipt (of payment; oft. hand-written)
はがき
postcard