I would suggest looking into those 3 books jfraser, but just to put it out ther, what my teacher has been teaching me so far, as far as rooting, is that its not as important to do the breathing through the bubbling well, or even use your imagination to develop a root. The Qi is already there, you just have to sink it properly and stay sunk.
I have been working on that method (Earth/4 gates) for 4 years, and my teacher could easily move me in my classes. Well he made a slight adjustment to my posture and had me sink more, and he couldnt move me. What he showed me is GB 31 is the secret. You sink with your hands at your sides. When GB 31 pushes out your middle finger from the natural bend when sinking, that is where you root is strongest. Sinking is not just bending the knees. You bend your knees slowly until you feel GB 31 push out your fingers, then you slowly let your lower dantian float downward as far as it will go. You will notice your huiyin move forward and up unconsciously. Once you can move down no more, thats your strongest root. Your legs should feel as if they support 1000 lbs, and your feet will feel as if they sunk into Earth. You'll feel like a tree. Your rooted feeling will connect to your hands, and if you shift wieght from right to left, the 1000 lbs will shift like water.
To help develop my root further, he has me sit in horse stance in that manner, and shift wieght 100% right, then left, then return to horse. He has me hold each position for about 9 seconds each. Then he has me do the Shaolin stepping routines staying at the same hieght throughout, holding each posture 9 seconds each, at my strongest root, and making sure my feet are in the strongest position to support an attack. ie push forward should be a bow stance, both feet point towards the opponent, wieght should be 100% on front foot. If you dont believe me on that part, or you were taught differently, try pushing an opponent head on with rear foot at 90% or 45% angle, then try pushing same opponent with both feet facing them. They wont move you the 2nd way.
Also you'll notice in this type of stance when doing a movement like part the wild horse's main, if you keep your eyes up and feel the point in the center of the palm, youll feel the front foot rooted, and the front hand, even though completely relaxed, will feel full and will be impossible for an opponent to move, no matter how much muscle strength they have. Beleive it or not, just feeling that point is the secret to "Aikido's" unbendable arm. It was in Taijiquan first.