They are credible, but do not demystify more than other textbooks.
I have worked my way through "Relativity Demystified".
I think it's a good book in the sense that it gives you a lot of tools how to do calculations in general relativity. No computer, no calculator, just pencil and paper. And in doing all the examples and the quizzes you really get the grip on, how to calculate a Riemann-tensor or Einstein-equation.
Regarding theory I would seek other sources. e.g. Carrol (2004).