Didymus wrote:D.R. Shackleton-Bailey's editions. The Select Letters in the Cambridge Green and Yellow series is a reasonable place to start; from there you can move on to his comprehensive scholarly editions (likewise Cambridge) if you are so inclined.
I second this recommendation, as I own it and have enjoyed reading numerous letters from it. The notes provide adequate context on references and context, as well as occasional textual questions. The introductory material provides some background regarding Cicero's life and Latin letter writing.
I think the most interesting thing about Cicero's letters is the wide variety of styles he works with. He'll switch between a loose colloquial style with his friends, and he shifts back into ornate periods when writing to various political figures.