(Find me at 50 Watts Books.)

Milton Glaser, cover for The Baroque Inevitable
Groove Is In the Art is a growing archive which celebrates "an era when psychedelic graphics and pop art met the mainstream on instrumental and classical album covers in an explosion of line art and color." I've featured some of my favorite covers in this post.
The man behind the site, David of Jive Time Records, also documents his personal collection at Project Thirty-Three (a "shrine to circles and dots"), Symphonie Fantastique (gorgeous 1950s classical lp covers) and Stereo Stack (vintage LP stereo banners).
Project Thirty-Three has been featured on Grain Edit and iso50, but I had to get some of these sleeves on my blog. I recommend subscribing to all of David's sites, especially if you're interested in graphic design—the big names turn up frequently, and almost always with forgotten work.

Herb Roguff, cover for Pierrot Lunaire

Cliff Condak, cover for Subotnik lp

Barbara Nessim, cover for Chopin a la Moog

Cliff Richards, cover for Carnival of the Animals

Herb Roguff, cover for Shostakovitch lp

Sandy Hoffman, cover for Works for Trumpets, Organ & Orchestra

designer unknown, The Cowsills and co, Pickwick Records
Plus three faves from Project Thirty-Three:

George Giusti, cover for Amor Mexico - S.A. (1966)

2!2 (EMI, 1973)

Josef Albers, cover for Persuasive Percussion, 1959
And one lovely Erik Nitsche cover from Symphonie Fantastique:

Erik Nitsche, Medee (Decca, 1953)
RIP Erik Nitsche flickr archive