The letters from writers are especially interesting, as they give us a glimpse into their innermost thoughts and feelings in a way their professional work might not. - Jason Diamond
Diamond brings up an important point here. As readers of this blog know I am a great fan of the writings of Virginia Woolf. Her writings, were for the time new, exciting and wide reaching in their subject matter. Her letters shed light on why she wrote as she did and exposes how she was feeling, and what was happening in her life, while the book in question was being written. This insight, which we don't normally see for most books we read, gives us more than a glimpse into the way that Virginia Woolf's mind worked.
For this reason I love to read collections of literary letters. I am currently plowing my way through Volume 1 of the Letters of Robert Frost.