I can I use a BI Publisher form field as an XSL Variable?

One of my (many) frustrations with BI Publisher is in understanding how the "normal" form fields can be used with and by the underlying RTF code.  And, to be honest, I really do not know exactly what programming language or syntax is meant by "underlying RTF code".  I assume that this is what is meant by "XSL-FO", but I am still very unclear how these two elements (BI Form Fields and XSL) interact with each other.  If anybody knows of a good book out there that provides a COMPREHENSIVE understanding of how to do XSL coding I would be very