A lovely big sister gave me this book for my birthday last week. It's a sumptuous history of sleeve design, from the 1940s through to the present. Filled with page after page of gorgeous album art, it also offers loads of background info on the evolution of the picture sleeve, as well as stories about individual LP covers.
The Art Of The Album Cover - Richard Evans (Compendium, 2010)
I highly recommend it to any lover of popular music, as well as fans of art and design. It can be found here.