There are ninety-two extant letters, seventy-seven of which are divided into ten books (only nine of which survive, numbered I-II and IV-X; the third book, along with the end of the second and the beginning of the fourth, has been lost from the manuscript tradition), with an additional fifteen extra collectionem. The first nine books include personal letters in the strict sense of the term, together with epistles on biblical and theological subjects, while the tenth book is devoted to letters of a political nature. On the contents and arrangement of the letters see Zelzer 1990, who argues that the ten-book division--modelled on the letter collection of Pliny the Younger--goes back to Ambrose himself. Zelzer (CSEL 82/2.XVII-XXXVIII) surveys the individual letters. For a summary of the major historical topics covered in the letters, see Mara (Pat. 4.176-77).

For collation of the PL and CSEL numbering, see CSEL 82/3.XIV-XVIII; PLS 5.393-95. References below will be to the CSEL numbers with the corresponding PL numbers in brackets.

Zelzer (CSEL 82/4.346-51) provides a list of manuscripts listing the letters found in each. The Royal manuscript contains books I-IX (letters 1-76), together with letters 70-77 of book X (see CSEL 82/3.CLXXX). The arrangement of letters here (see Warner and Gilson 1921 1.124, whose numbering unfortunately follows the fifteenth-century edition of Amerbach) with letters 75 [21], 75A [21a] (= Sermo contra Auxentium de basilicis tradendis), and 76 [20] numbered as lxxvii-lxxix, followed by DE OBITU THEODOSII numbered as lxxx, is found also in the twelfth-century manuscript Cambridge, Trinity College B. 3. 10, from Christ Church Canterbury (see Mynors et al. 1991 p 70) and elsewhere (see Zelzer's list). This sequence is followed in the Royal manuscript by letter 77 [22], numbered as "lxxxi," and DE NABUTHAE

The seventh-century Boulogne manuscript contains only letters 64-68 [74, 75, 78, 80, 26], with nos. 67 and 68 treated as a single letter. Despite its antiquity, this manuscript transmits a corrupt text of the letters (see Zelzer, CSEL 82/2.LIX). The contents of Oxford, Bodleian Library 137 (not in Gneuss [HG], but saec. xi ex. according to Gameson 1996, p 155) are the same as the Boulogne manuscript, including the group of letters letters 64-68.

The four Oxford manuscripts all contain (or once contained) extra collectionem 14 [63] ad Vercellensem Ecclesiam (the work has been lost from Bodley 804; see the entry for DE SACRAMENTIS On Bodley 94, see the entry for DE BONO MORTIS; for Bodley 516, a ninth-century North-Italian manuscript which came to England by the eleventh century via Brittany or Wales, see Webber 1992 (p 79).