There is an excellent review of three recent collections of letters by stalwarts of the faith at The Shepherd's Scrapbook. All published by Banner of Truth in the last two years or so, the works include The Letters of John Newton, The Letters of Thomas Chalmers, and The monumental , Letters of Samuel Rutherford.
I have found all three of these handsome volumes enormously helpful. Indeed, I have returned to them again and again for refreshment, Gospel hope, and pastoral vision. Despite the fact that at times the prose in these letters is far richer than that to which we have become accustomed, I highly recommend them.