I've changed my strategy to skimming letters written after publication of ROTK. I found this in Letter 165(p. 221) to Houghton Mifflin Co.:
Nothing has astonished me more (and I think my publishers) than the welcome given to The Lord of the Rings. But it is, of course, a constant source of consolation and pleasure to me. And, I may say, a piece of singular good fortune, much envied by some of my contemporaries....envied by some of my contemporaries
Nyah,nyah!! (shameless goating!)