Ivy, I am a great fan of "range." That is using the horse in ways that condition range of motion throughout the possibilities for that horse at that time. There are no exercises I see in AND ground work or in mounted work that do not apply in some way to this consideration.
If one is not pulling the horse's head up into unnatural positions, and is asking for motion, both in ground work and mounted the development of the neck-backband group is going to take place.
It needs, of course, not just vertical work, but horizontal work that calls for lateral exercises.
That especially, I think, is what to look for in the groundwork and other related folder/forums at AND.
Don't be intimidated, nor that any of this is over your head. It most certainly is not.
Before AND there certainly were effects on the horse that strengthened the neck-backband. It's just that here we approach it in a different way.
My own preferences as I teach others to develop their horses is to focus on forehand and hindquarter turns in place with little or no rein aid. Then circle work, with attention to the horse moving his head and neck freely and encouraged to lower the head, at least below the level of the withers.
I like and encourage loose rein work, whether the rider is using a bit and bridle, a bitless bridle, or hopefully no bridle at all -- so no reins.
I come from (though it's not directly AND related) the Caprilli school of cross country and jumping work, where great freedom to the horse's head and neck is encouraged not just by the hands, but by the seat as well.
The green horse is apt to raise his head and hollow his back, unless his groundwork has encouraged more development of collection and extension - range of motion and range of frame.
The reason I'm not specifically answering your question as to what to look at and read is that there is simply too much and the subject too broad. You can begin almost anywhere, but moving toward the beginning chronologically will probably serve you best.
The founders of AND put a good deal of work into the early forums on related subjects.
You will want to read the one article and thread that I will give direction to. It is on the "neck-backband," or in German, "Nacken-RÃ¼cken-Band." Romy felt that to an English speaker it might look odd, but I'm comfortable with it even untranslated to English as it so aptly and precisely, as German often does, describes the group of muscles, ligaments, and I think tendons involved.
You may or may not have read this thread, but if so, I'll offer it again. As you read it I think you'll begin to see how a great deal of what we do already addresses the flexing and strengthening of the Nacken-RÃ¼cken-Band.http://www.artofnaturaldressage.com/forum/viewtopic.php?f=7&t=621&hilit=nacken
But to be aware of what we are doing is the real trick in all this. We can better organize our plan, our routines with our horses if we understand better what is happening anatomically as we ask things of them.
It's quite a bit to think about as we are actually working with our horses, but hopefully we will find the time to take the time that it takes. Karen has what to me is a wonderful skill of isolating tasks, that is "chunking down," to manageable bits in her training. It's were I often go, Karen's posts, when I find I am just not able to focus enough for my horse. Or in fact for my students.
You'll find both Romy and Miriam, in the thread I've linked to above, go to considerable length to clarify what is taking place around this muscle ligament group. They certainly helped me better understand and appreciate the issues in conditioning the horse, and especially what my bottom had been telling me for years about the horse's back and it's need to rise and to stretch and contract in motion.
I tried to contribute some to the thread myself by citing an authority I have come to trust and respect.
Once you understand better how the Nacken-RÃ¼cken-Band works and really what it is I think the ideas will flow for you in planning your conditioning of your horse.