This third and final volume of the Acton–Simpson correspondence, somewhat larger than the other volumes, contains 328 letters, from July 1862 until Simpson's death in 1876. Over three-fifths of the letters date from the period of Acton's editorship of the Home and Foreign Review, until its suppression in April 1864. The correspondence is intermittent after that date, but there are spurts of activity in connexion with the publication of other periodicals, Acton's candidacies for Parliament, Simpson's researches and the Vatican decrees controversy of 1874. With this volume, the publication of the known surviving correspondence is complete, and the ghost of Gasquet may at last be laid to rest.

After the first draft of the edited correspondence had been prepared, further research by Professor Holland uncovered some twenty hithertounknown letters in the Shropshire County Council Archives, Shrewsbury. These have been inserted in the text without renumbering the original letters, and they are indicated by a capital letter following the number (e.g., 694A, 734A, 734B, etc.). Two letters from the period covered by Volume II are given in an appendix, as is the letter from Mrs Simpson to Acton announcing her husband's death.

There has been a change in the specific responsibilities of the editors, occasioned by Father McElrath's assumption of the office of President of St Bonaventure University. He was responsible for the preliminary preparation of the text, but the final editing was undertaken by Professor Holland, who also prepared the Index.