I love collections of authors' journals and letters. Even if I don't like a particular writer very much, or their work is unfamiliar to me, I'm still interested in reading any collection of their personal thoughts or correspondence. Sometimes, I'm more interested in a writer's thoughts to themselves or their friends than I am in their actual creative work. I just love being in people's heads, particularly if I have a collection of journals in which the writer is focused on their creative process - I'm fascinated by the ways in which people create and revise.
I have copies of Plath's Journals, Woolf's Writer's Diary and two volumes of her Diaries, plus I'm right now reading The Letters of Vita Sackville-West to Virginia Woolf. I have Rand's Journals and Sarton's Letters. And I plan to expand my collection as I go.