I was a little disappointed that you had to use the wooden bookshelf in Delicious Library when viewing your items, and that no other options were given. However, I found that it's really simple to change the look of the shelf by editing two .tif files in the contents.
Navigate to your copy of Delicious Library, control-click on the application and select "Show Package Contents" from the contextual menu. Then navigate to Contents -> Resources. The two main files that create the shlef are 'shelf.tiff' and 'shelfbackgroundtexture.tiff.' You can back these up (if you wish, but highly recommended), and then edit them into your own creations.
I just replaced them with the metal shelf files already included:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20041115123339229