The answer - all of them are good.Emacs and Org manuals are available from inside of Emacs with every installation. They describe out-of-box capabilities of Emacs and Org respectively. Ideally you want to skimp through them, so you'd understand what's available. Then you can reference them as needed.
Mastering Emacs is a tips and tricks book. It's good, but no replacement for the manuals.
-- Best Regards, Nikolay Kudryavtsev
