Hmmm, let me think...
I'd have to inspect the .mpb (sp?) files to see if/that they keep a log of the last access time, and compare it to all the others to confirm it was the last book read.
That might be hard because a lot of people read a ton of books at the same time. But, theoretically, I think it's possible. It would have to, in real time, parse/extract the book cover from the last book and dynamically create a png/jpg of it with the proper formatting and put it in the (hacked) "screensavers" folder on the file system.
I think that would be really cool; I'll see what I can do. I also think it would be boss if the screensaver could be a real-time clock, or something like that