Hey Derek,
Welcome to the 43F boards. My dad's a preacher, and he's actually fairly organized for someone with an undiagnosed case of ADHD. I think the most important thing for him has been using outlines when he preaches. Otherwise he'll meander all over the place. He has a neat little trick: He prints out his outline, cuts it up into roughly 4 x 4 inch pieces, and paperclips them in his Bible. He's been doing that for years.

Otherwise I think the best thing might be to make lists: lists of possible sermon topics, issues to talk about with the elders and other staff, prayer lists, lists of people to visit (could be grouped by area), calls to make, building maintenance, etc. GTD-itize that by contextualizing those lists, and review them as often as needed.

The more I do GTD and the longer I read the board postings, what makes one person's system different from another's is the contexts that are used. Like for Merlin, "@Computer" would be ridiculous since he's online virtually all the time. And for a salesperson who earns a living making sales calls, "@Calls" would be too much, so their Calls list should be sub-contexted by type of calls. And then some people have contexts that wouldn't appear on someone else's system.

So my point is, your dad could apply the GTD concepts by looking at his contexts and going from there.

As for reference material, and I'm sure he has a lot, he could group all his books by topic since there's a chance he won't remember the book's title. (But then with preachers, you never know. My dad remembers the phone number of almost every family in the 800-member congregation.)