Regarding continuity, I'll certainly make an effort to avoid contradicting other books in the same series, if I know about them or my editor alerts me to an issue. But I'll admit that there's a bit of sliding scale there. I wouldn't want to contradict a Dave Mack novel published last year, but I probably wouldn't throw out an entire chapter just because (gasp!) it clashed with one paragraph in a Gordon Eklund novel published thirty years ago. You need to keep "continuity" in perspective.
My general rule of thumb, and this is more of personal preference than Official Policy, is to not invent something new up if another author has already beaten me to the punch. So the description of Khan's flag in my Eugenics
books was lifted from an earlier novel by Jeff Lang, and I refered to Gary Seven's mysterious sponsors as the "Aegis" because that's what Howard Weinstein called them in an earlier DC comic book. And so on.
The way I see it, it's just a matter of professional courtesy, as well as a fun little easter egg for the fans . . . .