Thanks, General Relativity from A to B sounds like the book I've been looking for. There are lots of books and articles about the strange things, like time travel, that relativity suggests are possible, but they never explain basic ideas like spacetime and worldlines in much detail.
I don't have an extensive physics background, so I apologize if there's a well known or obvious answer to this question.
I'm familiar with the Newtonian version of time in which it's treated as a kind of fourth dimension. So there's x,y,z and t. Sitting here typing this, time is passing for...