Actually ideas are everywhere. It's the paperwork, that is, sitting down and thinking them into a coherent story, trying to find just the right words, that can and usually does get to be labor.
I doubt I'll ever do another book collaboration; I've been spoiled. Roger and I both happened to move to New Mexico at about the same time, when we each had a family of young kids to raise. Socializing seemed to lead naturally to working together.
More immediately, I'm currently working on another Dracula in which there will be connections with ancient Egypt. That's about as far as I want to go in commenting on current work.
The same tools that make any writer good, plus a cheerful willingness to suspend belief.
If people ask me for the ingredients of success, I say one is talent, two is stubbornness or determination, and third is sheer luck. You have to have two out of the three. Any two will probably do.