noun
collection of letters; collected letters
Refers to a published or compiled volume of correspondence, often by a notable figure.
漱石の書簡集を読んで、彼の人間性に触れた。
I read Soseki's collected letters and got a sense of his humanity.
往復書簡 refers to exchanged correspondence between two people, often published together.
Compound of 書簡 (letter) and 集 (collection).