collection of letters; collected letters
epistolary style; style of written correspondence
novel written in epistolary style
the Epistles (of the New Testament)
the epistles of Paul; Pauline epistles
Pastoral Epistles (i.e. Timothy 1 and 2, Titus)
General Epistles (James, Peter, Jude, John); Catholic Epistles; General Letters
Letter on the Blind (1749 essay by Denis Diderot)