This is all about skewing images. You'll need a program such as PhotoShop which can use layers and has a skew option.
Start with a normal version of your cover. Put this on a layer in your program.
On a lower layer, paste in a template of a 3D book. You can use my mockup above, or buy a "real looking" one from iStockPhoto. Heck, you could take your own photo of a lovely book in your own library!
Select the cover image. Resize it so the left hand side of the cover matches the spine of your stock image.
Now click on the top right anchor point of your cover. In the lower menu bar, choose the skew option. Drag that spot to where the cover ends on the base image. Do the same with the lower right anchor point.
Now your cover is set! It's just time to add words to the spine.
Type the title in, using a font and color that matches your theme. Rotate that text so it's at a 90 degree angle.
Now use the same skew tool to give a slight angle to the text.
Repeat with the author name.
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