Much as I appreciate today's major update to the Kindle App for iOS devices, I am rather disappointed that Amazon haven't added the collections feature which I find one of the most useful aspects of the Kindle Reader. This allows you to create collections (folders) where you can store categories of books you own. I keep it simple – the home page of my Kindle Reader contains the books I intend to read in the near future; a Finished collection is where I put all the books I've read; and a Reference collection contains dictionaries and other stuff I might want to refer to. It's simple but effective.
According to the Kindle help pages, collection information is stored on the Whispersync server and synchronised with other registered Kindles.
On the iOS apps, however, there appears to be no way to create collections so you have just two choices – books on the device or deleted books which are stored on Amazon's server. As a result, I use the Kindle Reader for organisation of my library and keep the bare minimum of books I want to read on the iOS devices or the Mac.
Kindle App 2.5 brings a number of innovations, including import of compatible files via iTunes and background downloading of books even when the app has been exited. A more detailed review is available here on MacWorld.