My Sony PRS-505 I put as many books on it as it will hold.
My Kindle has all of the books that I've purchased from Amazon, minus the ones that I have read. I have been reading my books in the order that I aquired them and my device preserves that order while the archive does not. I have not filled it up yet.
My Sony T1 only has the books I'm reading at the moment. It is *supposed* to replace my 505, but it does not organize my books the same way and I actually prefer the way the 505 manages my books compared to either of my newer readers, so I use the 505 to decide what to read and then I upload only that book -- or series -- to the T1.