I do have the MEC book and a number of other shooting specific and general sport psychology manuals. They all have useful advice and ideas that may help the shooter. However no amount of psychology will make up for a deficit in your shooting technique. In short, if you don't train hard enough then sitting thinking about it is not going to turn you into a winner. What mental training can do is help you keep your technique intact under the pressure of competition.
Rather than spend a lot on the MEC book, look for a secondhand copy of Sporting Body Sporting Mind on Amazon or ABEBooks. It is a good general book, well written and easy to understand.