I found this tecnique very useful. I didn’t have to photograph the work, keep it very consistant in size and style, and it looks realistic.
It is very easy to do. I created a jpg image of the DVD case design, opened it in Photoshop again and then just scaled it using “free transform” (Command+T or CTRL+T)
I was doing a web gallery of DVD cases and had trouble keeping everything very consistent, because when I was photographing the products it was hard to keep everything in same style. But making my DVD cases through Photoshop all my products were perfectly consistent in size and style.
Below you can see a very easy process. You can follow same steps with your book or DVD case design same way I did with basic shapes.