The Earlier Letters of John Stuart Mill, published in two volumes in 1963, were well received by critics and scholars alike. The publication of these four volumes of later letters completes this edition of Mill's personal correspondence. These volumes contain over 1,800 letters, most never before published, and some sixty earlier letters that have come to light since the publication of the first two volumes of correspondence. The letters have been assembled from widely dispersed collections in the libraries of fifty-eight institutions and of some thirty private collections in Britain and in other countries of the Commonwealth, Europe, and North America. In addition, many personal letters of which no originals survived have been located in contemporary periodicals or biographies of Mill's correspondence.

The collections of the Advocates Library, with the exception of its legal books and manuscripts, were given by the Advocates to the National Library of Scotland in 1925.