Hi Dany. I have read alot of Jared Tendler books and I think the mental game is very important. We can all learn strategy, but if we are not feeling right or comfortable, then it can all go to "pot" and normally not your own!! hehe
I think that some days I could beat anyone in the game - Phil Ivey, bring it on!! But some days I couldn't beat next doors cat, its just the way it is. The art is to learn to have more good day than bad, and that's where your "mental" game comes in.
I think the mental game of poker is a good book, and I havent only read it once, in fact I am reading it at the moment. It's one thing that I has helped me, and I am sure it will help you an others too. Good luck. Marc