Craig:
There is, of course, the classic book "Essential Data Flow Diagramming" by Menamin and Palmer.
The basic idea is to assume a world of perfect technology. For example, a business has a requirement to, for example, calculate sales tax on a purchase. In a world of perfect technology, a majical button would suddenly appear to the user. And the user would simply press the majical button and the sales tax would be instantly calculated: No programming to implement sofware, no database links, no servers - nothing but majic. (Of course, one can argue that in a world of perfect technology, the button would not even be necessary, just telapathic though, but I wont go there.)
The requirement to calculate sales tax is essential - it is technology independent and is a requirement even if the business has no computers. All the implementation requirements are non-essential - they exist only because of imperfect technolgoy (i.e., the lack of a majic). What the BA needs to do is come up with a comprehensive, integrated understanding of the essential functions and, especially, how they all interrelate. This then gives the logical framework into which all implementatiion considerations are properly pigeonholed.
Craig, the difficulty in understanding the concept of "essential functions" is that the BA needs to be really counter cultural. The BABOK , for example, teaches things like there are system requirements - which occur within a software system, and there are business requirements. Not true: Some essential requirements (i.e., business requirements) are implemented by computer, and some are not. This confustion results in questions like "What is the difference between a business requirements spec and a functional requirements spec?"
Tony