We have a library of technical documentation about Infinity and our internet products and that page can be found here: https://www.blackbaud.com/training-support/support/howto/infinity-sdk-resources

Right now, portions of the content would solely pertain to BBNC's big brother counterpart that integrates with BBCRM instead of RE - Blackbaud Internet Solutions (BBIS). However, several articles are excellent resources for learning about the BBNC API and how custom part development would work for either BBNC or BBIS. 

Blackbaud also offers customization training geared just for the developer audience in case you'd like a guided approach to learn about customization either BBIS or BBNC. Below are the 5 most relevant articles for those folks looking to customize BBNC specifically.