In order to use Mobipocket e-books, it seems I have to register a PID. What is a PID, and why do I have to register a PID with this site?
A PID (Personal IDentifier) is a unique identifier for your copy of Mobipocket Reader. Registration of your PID is required so that your copy of Mobipocket is registered to your device, enabling your device to open and use e-books that have been DRM-protected using DRM (Digital Rights Management). This process is necessary because many publishers require strong DRM protection to prevent unlawful redistribution of their e-books. Once your PID is registered, you can download and enjoy the many premium e-books that have been secured for distribution in this format.