Here's a great article from The New York Times about creating derivative works from existing products. Although that sort of thing happens all the time, the key here is to think of the e-content opportunities. Creating compilations, greatest hits, and similar products in the physical world involves a cost-of-goods component which not only increases the expense but also adds to the risk. That's not the case in the e-world.
Regardless of whether it's for the Kindle, Sony Reader or some other device, every editor and publisher should inspect their product lines for new revenue streams like this. And while the starting point is your existing content inventory, I think it's even more important to use this as a lens through which to view your future content acquisition plans. How might those plans change if you're able to create additional derivative works with them?