This is a set of two plug-ins: one creates a new template file, the other combines that template to create a '3D view' of your ebook cover.
These plug-ins require Python! If you have the menu item 'Filters->Python-Fu->Console' then you are ready to install these. If not - search the forums or the web on how to add Python support to your version of GIMP.
The 1.0 versions are functional, but have a lot of room for improvement. For example, the combine plug-in makes use of the 'map to box' plug-in which takes quite some time on my quad-core - if you have older hardware, this step may require coffee ;)
Part 1 - The Template
Part 2 - The Ebook
I've uploaded both plug-ins here, as well and an example template XCF with generic artwork added and the resulting PNG file. Example files are zipped.