diplomatic message sent by a head of state; sovereign letter
book written in Japanese (as opposed to Chinese, etc.); Japanese book; national literature (of Japan)
大使は国書を提出して着任した。
The ambassador presented credentials upon taking office.
国書の交換により外交関係が始まった。
Diplomatic relations began with the exchange of sovereign letters.
国王の署名が入った国書が届いた。
A sovereign letter bearing the king's signature arrived.