Sentence:  Ch‘ing Twan-hëoh... took his third degree in 1,321. He was much employed in the office of historiography, and composed the Work next mentioned and another on the Ch‘un Ts‘ëw before he felt equal to this, which is reckoned his <WSD>"chef d'œuvre</WSD>". 
Meaning of the word marked with <WSD> tags:  A piece of work that has been given much critical praise, "especially" one considered the greatest work of a person's career.