I found a copy of letter 246 here. I think it’s one of the most important letters because it reveals some of the ideas and beliefs that were fundamental to LOTR.
BTW, this letter is a draft found in Tolkien’s papers - it was never sent.
The letter includes Tolkien’s outline of what would have happened if Gollum had not taken the ring at the last moment and Frodo had kept it (in essence, with the help of the Ringwraiths, Sauron would have easily recovered it). He also describes the power of the Ring and Sauron’s power even when someone else is wearing it.