Can anyone recommend a (simple) database that would work for keeping tabs on characters across a series of novels, plus plot details and threads?
Some people use Bento. But there’s also an application called Scrivener that’s rather good I’ve heard… 
(Set up folders for Characters, Locations and Events in the Binder, create templates with questionnaires or attributes, and transfer from project to project with drag-and-drop.)
Naturally, as a complete Scrivener fanatic, who’s written my last four books entirely on Scrivener, that’s what I first tried to do, but I can’t sort and cross-reference and work out the threads in the way I need to. Thank you for the Bento suggestion.
You might consider Tinderbox, very powerful program, and you will find some discussion of this very topic on the Tinderbox Forums