I recommend Pro Drupal Development. It's the most recent (last time I checked) and has great examples and easy-to-follow narration.

@ TCS - testing the latest code on our test box, everything seems to work great! I can import our old certs and it seems to recognize everything as it should. Kinda wish we had the certificate entry code on the checkout page, but that's okay - I think attaching it to a user is cool!

But, suppose an admin wants to send a certificate to a user not on the system. Seems you can only do that if you are purchasing a certificate, but not administratively sending one. Any thoughts on that? Or am I missing something?

Otherwise - great work! Will be implementing it soon