The books are in EPUB format.
The PRS-500 must have firmware version 2.0 to support the new Adobe® Content Server 4 (ASC4) Digital Rights Management (DRM) and EPUB format.
NOTE: If your Reader does not have firmware version 2.0, call customer support at 866-962-7669 for assistance with updating the firmware.
The PRS-505 requires a firmware update to support the Adobe DRM and EPUB format. The update is available through the Drivers & Software link on this page.
All other devices support EPUB format natively.