Ego--not just your vanity, but all the things that are wrapped up in your sense of identity and that can therefore make you feel vulnerable and defensive--is a huge roadblock on a horseman's journey. So a few months ago when a friend mentioned he was reading a book about dealing with your ego, I said, "That's exactly what I need."
A New Earth by Eckhart Tolle is a sort of spiritual self-help book that is geared toward helping you be in the present moment without being mired in excessive analysis or emotion. Tolle's book is unlike anything I've read in its immediacy: rather than just telling you how to change your focus, the process of reading the book itself changes your focus so that you feel lighter and happier just by reading it.
And it's absolutely what every horseman needs. It helps you understand how to be more like a horse by living with the horse in the present moment. And it gives you a road map out of resentment and frustration by showing you how to get outside of the stories you tell yourself that create those emotions.
In that sense, it builds on Linda Parelli's strategy of stepping back and saying, "How interesting . . . ": rather than just giving you a strategy for when you're already emotional, it gives you a whole new way of looking at the world that--hopefully--helps you not to go there in the first place.
I know I'll read this book again and again if only because reading it is like an active meditation that makes your thinking--and therefore the world--a better place.