Copyright law used to be simple here. You could not use letters without permission. Period. For the most part, you still can't. But thanks to congress, you can now use excerpts from letters without permission, if the work is of a scholarly nature.
But using entire letters never works, and even extensive excerpts are not allowed except in certain types of books. Just two or three years ago, there was lawsuit based on a lawyer's cease and desist letter being posted online. The district court upheld the copyright on the letter, even though the poster was the one who received the letter, because copyright law states that the writer of the letter owns publication rights, and in particular owns first publication rights, which, of course, covers unpublished letters.
And as the U. S. Copyright website says, it's best to get permission for everything you want to use that you didn't write.
