Recent Imports is probably the least understood feature of Delicious Library, which is a real shame, because understanding it can help you keep your collection in top-shape, while neglecting it can lead to slowness, freezing, or even crashing.
Recent Imports is unlike any other shelf in the program. For one thing, it's got metal shelves instead of wood. Its contents and its sorting order are outside your control, and even though it's not part of your top level collections, deleting an item in Recent Imports deletes it for real.
Pre-dating "smart shelves," Recent Imports contains the last 25 items you've imported, sorted by reverse import order (most recent items first), regardless of which top-level collection they are in. It's also the only shelf that can contain items of an unknown media type.
Unlike a simple smart shelf, however, Recent Imports expands to contain any unresolved items. An unresolved item is an item that does not represent a media item. So, for example, if you create an item by trying to look something up, but the lookup fails, that failed item is unresolved, and everything you scan after it will remain in Recent Imports.
The reason is context. Instead of simply having a list of failures, you're able to say, "Well, it's the item I scanned right before I scanned this other item, so it must be..."
When you take care of the unresolved item, the shelf shrinks back to 25 items, unless there are other unresolved items, in which case, it shrinks to whatever it needs to to contain these other items.
If your Recent Imports contain more than 25 items, you need to find out why. Switch to shelf view (since you can't see the special covers in list view) and look for the following types of unresolved items:
Ambiguous Results: Paper cover with blue, two headed arrow sign
These are items that had more than one search result. The "Details" pane will contain a list of results. Click the correct result to complete the lookup, edit the search and try again, or delete the item.
No Lookup: Paper cover with three brown dots
Lookup Failed: Paper cover with orange caution sign
These are items that were never searched for, or were searched for, but were not found on Amazon, for whatever reason. The "Details" pane on the right will have the search terms. You can either edit the search and try again, delete the item, or press the "enter details" button. Even if you don't enter any details, pressing this buttons tells Delicious Library to stop trying to look up this item.
This last point is important. A lot of people will create an item and enter a title and creator and think they are done. As far as Delicious Library is concerned, this is just a search that hasn't been executed. You have to press the "enter details" button!
Note that the "enter details" button only literally contains the text "enter details" when the media type of the item is known. If the item was created from within Recent Imports, enter details is actually four buttons, each representing a different media type. Either way, the effect is the same.
Once this is done, close Delicious Library. It may seem to hang upon closing, but leave it alone until it closes on its own, even if it takes several minutes. Then, reopen Delicious Library and continue as normal.
In case you're curious, we're re-architecting the lookup system in Delicious Library 2 to replace Recent Imports with something that stands out a little more.