The back cover, aswell as the sleeves on the UK Children's edition of '
Half-Blood Prince', have been revealed.
They can be viewed
here.
The following is a description of what can be seen (highlight to read):
It seems to depict a pensieve in a cavern, with water and a rowing boat (there appears to be people in the boat, in the reflection of it in the water). The sleeves show two hands united with fire wrapped around them. These look to be the hands of Harry and Dumbledore. On the spine, at the very top, is a ring with an encrusted jewel in the center. The jewel appears to have a lightening-bolt shape in the middle of it. Bloomsbury have removed the blurb to prevent any spoilers being leaked.
Source:
The Leaky Cauldron 
Back cover of UK HBP (spoiler) 
