I wonder if you've worked at reading "Process and Reality" for its own sake, as a book that has its own, extra-theological purpose?
Did you feel it necessary to understand process philosophy as a whole, or did you abstract from it what was immediately relevant to theology?
The book to which he's referring is Alfred North Whitehead's magnum opus, a massive and famously dense systematic metaphysics. It's by far the least readable of Whitehead's works, which are for the most part quite accessible. In most of his books, Whitehead tended to write essays and chapters in response to particular problems, linking them together gradually to make a limited number of theoretical points, but staying connected to the examples and situations that prompted the exploration.
But in PR, as Whitehead scholars call it, almost nothing is connected to reality as we know it. The prose is dense with jargon -- actual occasion, prehension, nexus, eternal object, consequent nature. There are no examples. Every so often Whitehead will give a short discourse on other philosophers treated related ideas, but mostly it's chapter after chapter of philosophical heavy lifting comparable to Kant for sheer opacity.
Emmett, PR isn't for reading. It's for reference. Ideas that Whitehead tries out in Science And The Modern World or Adventures Of Ideas get constructed robustly in PR, defined and polished and placed in the matrix of all the other ideas. But the only way to find out why those ideas got created at all is to be introduced to them in the less speculative, more grounded works.
I tend to read the first and last chapters of PR straight through. The rest I work like a jigsaw puzzle, moving the pieces around and ignoring the ones that aren't part of the picture I'm currently working on. Or maybe like a textbook, studying the word lists and doing the exercises to try to get the one point that matters right at the moment.
In answer to your other question, I think it's a mistake to try to extract from PR only the theological content (which is concentrated in the last chapter and mentioned briefly in a few other places in the text). Understanding process philosophy as a whole is important, although I don't claim to have mastered it; nevertheless, I continue to try because I don't want to sin against Whitehead by failing to understand him as he presents himself, rather than as I might want to make him useful for my projects.
There are many -- perhaps most -- who are better at reading and using PR. But since you're asking me, I'd say you need a guide, for sure. I was first led through PR by my mentor Will Power, at the University of Georgia; then more systematically by the late Langdon Gilkey, at the University of Virginia. While I had already struggled through much of it myself in both cases, knowledgeable teachers gave me context that allowed me to situate the metaphysical solution in relation to the problems it was designed to solve. Everything makes more sense when it is nested in a network of relationships -- an observation Whitehead would certainly appreciate.
Thanks for the question, Emmett -- hope you came back to read it. For the rest of you, please forgive this brief foray into the more technical and less generally relevant (or interesting) aspects of my academic specialty. You're welcome to cleanse your palette by submitting a question of your own!