I am just learning to ride (hunt seat) and I have the Centered Riding book. So far, it has been very helpful. I think my instructor must be a fan, because she's taken me through some exercises that I recognize from the book.
The most helpful one for me (because I'm so new) was the one where she suggests seeing yourself as a tree with roots in the ground. That one has been REALLY helpful since the horse I'm riding right now has a tendency to jack around with me by pulling on the bit (even though he has an appropriate amount of slack in the reins, and I am not riding the bit, according to the instructor...she says he's just messing with me and playing games). Anyway, the "tree" metaphor really helps me keeps me from having my seat shifted when he yanks on the reins.
I am just learning to post (had my first big success on that front today) and the whole image that she provides for being pulled forward through your elbows has been *invaluable*. I'm sure she'll be just as helpful when I get to more complicated stuff!
Also, I do a lot of meditative work that has nothing to do with horses...a lot of her imagery has to do with what I'd consider getting "grounded and centered" - and that is important whether you're using for operatic singing, fencing, martial arts, gymnastics, zen, or - as she uses it - horses. Even though I have been using many of these techniques for years in other areas, it did not occur to me to use them on horseback until I read this book.