The 587031 is the Blue Sony PRS-300 Pocket (older model). You have to look in the Google Cache to see it online, they don't have it active on the website right now. The 586938 is the Pink one.
This both read standard ADE ePub files so you can buy books from the Sony eReader store, Kobo books, Books on Board and many other stores. Not Barnes and Noble,
Amazon, or the Apple iBook store.
They will also work with the Overdrive System many Public Libraries use.
They do not have WiFi or 3G, all books have to be bought/loaned via a computer and then sideloaded onto the device. The are a little smaller than the Kindle/Nook - 5" screens.
$99 is pretty much the Clearance price on this everywhere now, but since they are being Clearanced out they are harder to find.
Overall they work well though. I have one and my High School kid totes it around quite often. It keeps his hands off my nice Kindle.