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Mathematician's Delight by W. W. Sawyer (Pelican 1946). Cover by Robert Jonas. A playful illustration of math that makes a dry subject sound fun. Via grll8trax.
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The Comforters by Muriel Spark (Penguin 1963). Cover illustration by Terence Greer. Great cover illustration — you get a sense of what the character portrayed is like at a glance. Via grrl8trax.
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Zuleika Dobson by Max Beerbohm (Penguin 1961). Cover illustration by George Him. Charming illustration that's a perfect complement to the tone of the novel. Via jonathan229.
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A Horse and His Boy by C.S. Lewis (Puffin 1976). Cover and inside illustrations by Pauline Baynes. I read these versions of the Narnia novels when I was small, and loved the illustrations by Baynes — elegant, with a medieval flavour to them. Via scatterkeir.
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As I Lay Dying by William Faulkner (Penguin 1963) Cover illustration by Andre Francois. Interesting perspective, and nice details, too — look closely at the feet of the mourners. Via letslookupandsmile.
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Animal Farm by George Orwell (Penguin 1967). Cover illustration by Paul Hogarth. This cover really captures the menacing tone of the novel in a simple but effective style. Via letslookupandsmile.
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Gardening for Australians by R.T.M. Prescott (Penguin 1965). Cover design by Robin Wallace-Crabbe. Elegant and interesting approach — so many other publishers would have simply stuck a photo of something green and leafy on the front. Via letslookupandsmile.