How credible are the Demystified Series physics books by David McMahon?
They're ok, but definitely not for the serious student of physics.
They are credible but not so great in my opinion.
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).
The book contains quite a lot of typos, and I have published an extensive list on my homepage
http://physicssusan.mono.net/9035/General Relativity - Relativity demystified"
some of them are good, some of them contains TOO many errors and misprints to be valuable for a serious student. The QFT book has errors on almost every page...
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