Currently I'm reading Coverscaping: Discovering Album Aesthetics by Asbjorn Gronstad and Oyvind Vagnes - a book that's a truly critical study of album artwork.
As its authors note, many publications about sleeves tend to be of the coffee table variety but this has got some real academic depth. Personally, I always liked Adrian Shaughnessy's introductory text within Intro's Sampler series of books. This takes some of those sorts of discussions further while also delving into a whole range of other ideas about audio-visual relations.
Amazon link: Coverscaping: Discovering Album Aesthetics