雁書 is read as がんしょ (gansho) and means letter.
がんしょ
gansho
がん
Gan
かり
wild goose
ぶんしょ
document addressed to someone
としょ
books
どくしょ
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)