The letters of Robert Lowell and Elizabeth Bishop to each other. The first few pages were very, very subtle, with very subtle hints of romance, deep friendship, and somehow their infidelity. Their personal lives. But if you were for their perception on writing, they constantly critique each other and others in their circle like Auden, Howe, Plath, Allen Tate, etc. Though the core of their conversations are intrinsically personal, some insight on writing can still be reaped.
Some pages can be read here: http://www.solearabiantree.net/namingofparts/pdf/articles/travisanowordsinair.pdf