There is the 10-times reset methode to get the reader into fw-update mode.
reset, restart once the screen flash reset again, repeat
After the 10th reset a message fw-update incomplete should come up.
You can also try to reset the reader. Switch on and connect within the next few seconds to USB. Probalby a single removeable device with one file !proteced show up.
Then you can try to run the stock-fw-updater
Though I don't know if this is the "EPUB" enabled version. (and there is a PRS500.Rootfs+Fsk.2.0.00.09180.zip in aboved mentioned download location)
And here are some links found by googeling "Sony PRS-500 unbricking"