Thanks for your suggestions, everyone!
Re. Rites of Battle: Wow! You’re all near unanimous on this one! Just what makes it such an essential purchase? What material in it has made your games richer / easier than they would have been with the core rulebook alone?
I know it has vehicles, but after fiddling with vehicle combat in other RPGs, I have to say I much prefer personal scale stuff.
Re. Mark of the Xenos: I do feel as though the core rulebook is light on enemies. I’m one of these folks who feels as though his ability in mathematics and probabilities isn’t quite up to creating stats for new creatures, so I’d feel more comfy having an alamanc of adversaries created by those who know the system.
As to chapters and fluff… well, I’ve been down that road before. Back in the day, I used to collect the books for a game called Heavy Gear. I loved the pants off its sheer, living, breathing detail and its simultaneously epic and very personal metaplot.
Problem was, I spent so much time collecting, reading and studying that I barely played, and when I did, I was too tied up in making sure Everything Was Right to actually have fun GMing. My campaign lasted for about three sessions and then collapsed.
I’m a bit over that nowadays, which is why I’m largely happy to stick with the core rulebook. I also find myself subscribing to an idea I came across in a couple of indie RPGs, which is to enable the group to come up with their own fluff during play. I posted a house rule that I wrote up over here.
I’ve come to the belief that the group will be more interested in the fluff that we create over someone else’s idea that we had to pay for. As long as we’re on the same general page about the broader setting and mood, we should be fine.
I also think that if we have a bunch of PCs and NPCs who Want Something They Don’t Have, missions will write themselves.
So setting and adventure books, I’m not too worried about.
Oh, Adeptus-B: Yeah, I think a couple of my players have already splurged on the sourcebooks with the Chapters they’re keen on! I'm cool with them using them, as long as they manage the particulars themselves!