I have to confess I got bogged down in the collected letters of both John Steinbeck and Jane Austen, and so decided that maybe reading other people's letters was not for me. I tried it again with the letters of Robert Louis Stevenson, but again gave up after a while.
Then, in 2015, two books got me into reading other people's mail:
The first os Letters of Note, compiled by Shaun Usher. It is an eclectic collection, and it frequently contains reproductions of the actual letters. Most of the letters are very interesting, many beautiful, but my absolute favourite is a standard form letter from a Bureau of Etiquette in China, AD 856. "I was ready to sink into the earth with shame." Indeed. Wonderfull stuff.
The second one is Simon Garfield's To the Letter, an ode to (or obituary for) the letter, which covers the history of the letter from ancient Greece to now, the history of the postal system, various famous letter writers, and some letters from Chris Barker and Bessie Moore, who wrote to each other while he was stationed in North Africa during WWII, fell in love, carried on their romance by mail, and finally got married. (Garfield has since edited a collection of their letters, My Dear Bessie.)
I heartily recommend both books, and I plan to give collected letters another go. Maybe Kurt Vonnegut or Mark Twain will be the thing. We'll see.
Whose letters do you read?