Another new book called His Own Voice is set to be released! It contains his never-before-seen handwritten journals, as well as letters, notes and lyric sheets, and unseen photos. It's been compiled and edited by Jeff's mother and David Browne, who wrote Dream Brother, and there will also be an audiobook. Sadly it's not set to be released until spring 2019 though
More here: http://www.rollingstone.com/music/news/jeff-buckleys-handwritten-journals-to-feature-in-new-book-w473697