Correspondence with T.S. Eliot
While I've found correspondence between Ong and Eliot's representatives before, today I found a letter directly from Eliot himself, dated 30 Dec. 1947. It's nothing exciting, just Eliot thanking Ong for an earlier letter and the comment that he's looking forward to receiving an offprint of Ong's article "Wit and Myster: A Revolution in Medieval Latin Hymnody" (Speculum: A Journal of Medieval Studies 22.3 (1947): 310-47). I've also learned that we do have at least seven other letters from Eliot in the archives.
Personally, I've found the letters from Francis Yates and J.R.R. Tolkien much more exciting, but that may be because I knew from the outset that I should come across letters from Eliot. That the collection would contain correspondence with Yates wasn't surprising, but it was of great interest to me. Finding Tolkien's letter, on the other hand, was a real surprise, and I'm sure some of the joy in finding the letter is that I recognized Tolkien's handwriting before reading anything, let alone turning the card over to check the signature.