戰書嘅英文叫乜

A letter of challenge

To use it in a sentence:
Yesterday, I received a letter of challenge to declare for a boxing tournament with me on the followiong Friday.

戰書 (用作宣佈打仗的) = declaration of war

薦書(用作推薦工作, 入大學的) = letter of recommendation, reference letter

War declaration