Kurt Godel's Collected Works include his correspondence in volumes IV and V.

The end of Solomon Feferman's preface to volume III is an amusing historical document in itself, from 1995 -- I see it as capturing the brief moment when one could perceive e-mail as an alternative to physical correspondence, rather than a default:

"One feature of our work together [on the editorial team] in recent years deserves special mention: namely, the use of the electronic mail system, which has radically transformed and facilitated our communications and decision-making, and has produced a new art of nagging. How did we ever live without it?"