The William Tallack Correspondence is a collection of incoming letters, most of which are in connection with Tallack's position as secretary of the Howard Association, an organization devoted to the cause of prison reform. A tireless social activist, Tallack enlisted the support of authors (James Anthony Froude, Robert Hawker, Richard Houghton, William Lecky, George MacDonald, George Augustus Sala) and clergymen (Randall Thomas Davidson, John Jackson, Henry Parry Liddon, James Martineau, William Pennefather, Henry Richard, Charles Haddon Spurgeon, A.P. Stanley, Archibald Tait, Frederick Temple, William Ullathorne, Herbert Vaughan) as well as philanthropists (Michael Bass, Anthony Ashley Cooper Shaftesbury), social reformers (William Booth, Charles Bradlaugh, John Bright, Mary Carpenter, Frances Cobbe, Ouida), and statesmen (Arthur James Balfour, Joseph Chamberlain, Randolph Churchill, William Gladstone, John Wodehouse Kimberley, John Morley, Thomas O'Hagan, Charles Stewart Parnell).

Most of the letters in the collection are single items of correspondence thanking Tallack for supplying 1) copies of his letters about prison reform which had appeared in various publications; 2) the annual reports of the Howard Association; or, 3) Tallack's own books and pamphlets regarding prison issues, and others of his various religious and political tracts. Responding to the receipt of a reprint of a Tallack letter, Herbert Gladstone writes (3 Oct. 1906):

I hold as strongly now as in 1894 that our efforts shd be to prevent the young criminal from joining the ranks of the habitual; & that the habituals themselves should be dealt with by methods more rational & more deterrent than those now in force. And the very fact that so much has been done to make the prison treatment less repulsive & more hopeful is in itself a reason why hardened old ruffians should not be allowed to reap all its material advantages without the least intention of profiting or endeavoring to profit an the moral side.

A Quaker, Tallack received the support of many of his fellow social reformers who shared his religious convictions (Joseph Bevan Braithwaite, Bright, W.E. Forster). The earliest letters in the collection are from Quakers William Allen, William Forster, Joseph John Gurney, and Joseph Sturge, however these items are addressed to either Simon Horner or John Barry, rather than Tallack.