The Guardian has a recent article on the rest of Eliot's unpublished letters:
Karen Christensen, who worked as an assistant to Mrs Eliot in the 1980s, says that only half the material that was completely ready for publication in 1988 has in fact been published. The years 1922 to 1927, intended as the second volume, have never appeared.
Without the letters, critics have instead mined the poems for evidence, variously detecting homophobic, homoerotic, anti-semitic or just plain misogynist strains.
Ms Christensen writes: "The letters in the second volume were the most moving of all the hundreds I worked on.
"They catalogue the breakdown of Eliot's first marriage, the bewilderment and despair of two people who seemed unable to avoid destroying each other ... A second volume of letters would do much to reveal what really went on between them and would, I feel sure, create sympathy for Eliot."
(But one has only to read Four Quartets to see the sobering issues he was dealing with towards the end of his career.)