I've seen some of his stuff and personally, I think there are better (much better) sources. I don't like the guy's work myself. Pleasures of Small Motions, The Inner Game of Tennis, and even Zen Archery are good reads. I think his DVDs are a joke. But that's just me.
I'm gonna ramble a bit, but if it's helpful to anyone, then I did some good.
Now while I'm no expert, I believe your mental game grows as your physical game improves. If you're not sure of making the shot, or if you are not totally prepared to shoot the shot because pieces of the shot's puzzle are missing or unclear, everything else falls apart like dominoes. It may seem like mental lapse, but in reality it's confusion IMHO.
I mentioned this in another thread a little while ago, but I think it's very important. I believe that if you are absolutely sure of your shot. If you plan it completely in your mind. The part of the pocket to hit with the OB and the path / route the CB takes to an exact resting place on the table, then your mind will be much less likely to wander while you are in your stance. The mental picture is complete and there is no confusion on that last forward stroke.
My favorite line comes from Curly, one of the 3 Stooges. He said, " I'm trying to think, but nothing happens." It's truer than we realize. You can't think if you don't know what to think about. Or if you're not comfortable with what you just thought.
Sometimes you plan your shot well, see the line your CB will take for position, then get down to shoot the shot and realize the angle just isn't what you envisioned while you were standing. If you don't take the time to regroup, but rather try to adjust while you're down, things go wrong.
Another huge factor in the mental game is desire. For me, as I get older with a few extra aches and pains, my level of desire goes down and that effects the mental game a lot.
Outside distractions in our minds can range from anything from job, family, pets, God knows what else.
I believe if any of these kinds of things cloud our thoughts, no book is going to be the magic bullet. Rather, as I mentioned, filling your mind with the specifics of the shot, so long as your physical ability can deliver on your mental image, and being in the moment, watching your mental movie play out phyically works best. IMO