Here is (to me at least) an interesting site edited by Shaun Usher which every day features one or two new letters by a major historical figure in the literary, political or social world.
It also has occasional interesting letters written by lesser-known persons of historic note. Several recent examples of note have come from John Steinbeck, Charles Dickens, Douglas Adams, Nick Cave, Mark Twain, E.B White, etc.
I hope you'll have a look. :-)