There are good news regarding BOPF documentation.
As BOPF made it to the NetWeaver 7.5 stack (before it was part of the Application Foundation Layer of the Business Suite and therefore not available on plain AS ABAP), there is a BOPF Developer Guide on help.sap.com:
I hope that helps to make BOPF “the” common ABAP framework. As BOPF is also part of the S4 programming model it’s worth to start to explore the framework.
Furthermore, there is a regular podcast from the SAP Transportationmanagement (SAP TM) development team. As TM is also build with BOPF, they cover from time to time also some technical topics. The last two episodes covered BOPF:
Link on SCN: TMP: The TM Podcast