These two schools are entirely separate from World Building, that's something almost all Science Fiction and Fantasy Writers do, well, all the good ones anyway. Outline Writers and Discovery Writers both tend to get varying details about their Characters, Setting, Conflicts, and Rules Systems generally figured out before they begin, Outline or no Outline.
Outline Writers build complex, sometimes incredibly detailed outlines of their plot. This allows them to set scenes, character interactions and growth arcs, and general story progression in detail, usually before they put down their first paragraph. Working from a good outline allows them to work from point to point on a relatively clear path, even fleshing out individual scenes or chapters out of order. That first paragraph may in fact be nowhere near the beginning of the book. The Outline Writer is generally very organized and meticulous in their outline. It's a structured approach to their art.
Discovery Writers take the Characters they create, put them in the Settings, constrained by the Rules, and just follow along, literally discovering the story by seeing how their Characters react to the situations they're put in. They get to partake in the adventure along with the Characters, being surprised by their actions almost as much as the reader. It can be a gratifying and fulfilling method of writing.
Naturally, most fiction Writers have to fall into one, or a bit of both, of these schools. Myself, I'm finding that I'm 70% Outline Writer, 30% Discovery Writer. Having polished off a 10,000 word outline for BookB, I've found there's still a quite bit of discovery in outline writing, filling in the gaps between major plot points that I want in the story arc, turning the act of writing the outline into a discovery process.
There are other sorts of hybrids that I'm aware of; Discovery Writers that discovery write more than half a book, then outline the rest to ensure that it actually ends up somewhere and Outline Writers that do incredibly basic outlines and discovery write the rest.
Concerning BookB itself, after getting the Rules System sorted out for the magic of the world, a great deal of the previous Outline became either broken or felt weakly constructed. So, with the new tools of the Rules System worked out I set to recreating the Outline of BookB. I still had the same basic premise, same characters, and overall plot, now I had a strong set of building blocks to work with and everything came together much tighter. I'm sure in the actual writing process things will become more refined and cleaner still, though I fully expect there to be several revisions before I'm happy enough to give it to a Reading Group.
Now to dig in and go beyond a few test paragraphs, which I use to help find a Character's Voice, something I find quite helpful and important, and to build scenery and mood. I'm getting excited to see how this goes. I'll keep you posted.