The Selected Letters of Thornton Wilder Book

I've read all six volumes of Mark Twain's letters and although some obviously are more interesting than others, they are a must for anyone interested in Mark Twain, the man.

These letters give us the background to his works and show him to us as a complex personality with very pronounced weaknesses and strengths : his deep and constant love for his wife Livy, his great capacity for true and loyal friendship, his impetuosity, his restlessness, his extravagance, his occasional childishness, his impatience, moodiness, vanity, generosity, tolerance, honesty, enthusiasm, etc. etc.

I also found Albert Bigelow Paine's annotations very helpful.