Malika Favre's picto-porno-typographic cover for the Deluxe Classics edition of Vatsyayana's Kama Sutra, which I imagine will wrap all the way round the back and onto the French flaps. The red slashes of the gloves and stockings are the perfect extra element which makes this, I think.
See also Favre's rather more Sapphic variation on this, from her site (click for a bigger version, as the actress said to the bishop):
Then there's Max Ellis's cutaway cover for Titanic, First Accounts, another Deluxe Classics also making full use of the flaps. Here's the front and the full fold-out cover (sorry for the low-res version of the latter, but it's all I could find).
Here's a Max Ellis alphabet, vegetable in theme:
And finally, designer unknown, this simple but eye-catching cover for the great Nikola Tesla's autobiography, using the best handsome-devil photo of him that I know of.
Bonus content: The Doctor Who theme played on twin Tesla coils...
..and this great photo of Mark Twain in Tesla's lab, 1894...
