It looks fantastic, there are however a couple of things.
you are using 3 different logo's fonts for the turrican writing, the one on the back is obviously an ingame shot, but the front and spine should be the same logo
(where is the spine logo from, I cannot find it in any amiga screenshots etc.
Try and avoid using screenshots as logo's etc in general as the 320x256/200 size doesn't resize too well, and looks not so good when it is (again spine logo )
An idea.. why not create a spanning spine set version too, that would indeed be awesome