I've seen a few people lately asking where they should submit book covers when they are uploading them to dA. First, they should NOT be submitted to Digital Art > Photomanipulation unless they are simply the image (e.g. they do not have an author, title, etc.).
So, where do they go if they are actually book layouts (with author and title)?
You can submit them here: Designs & Interfaces > Editorial Design.
I hope that helps.