I have not read the book mentioned, so I do not know what the recommendations in it are. I can tell you, though, that you do not feed all horses the same. There are as many differences within the species as there is between animals from different species.
You have to feed TB race horses and other hot-blooded high performance horses a lot differently than an easy keeping saddle horse. You have to feed a draft or draft cross much differently than either a lean, hard-keeping TB or a easy going saddle horse. Then, there is fitting and conditioning horses for shows and halter classes. All of these differences are not even related to the metabolic problems specific to horses. That adds an even more 'horse specific' dynamic to choosing the right feed for each different horse. Some horses need a LOT more calories than others. Just like people, there are horses that can eat everything in sight and stay thin and others get a few pounds of hay and no grain and stay too fat. Obviously, you have to feed them differently.
So, there is not a right or wrong way to feed a horse. There are many different ways to feed for each purpose and for each body type and digestive system because there are many different kinds of horses and goals. You obviously don't want a saddle horse as 'high' as a race horse, either.
You would never want to feed a race horse low protein mature old grass hay but, it is just the ticket for a saddle horse. If a horse is 4 or under, I would want to add a little soybean meal to the low protein grass hay and would sure want to add Ca. I would only add grain or another concentrate other than the soy if that particular individual needed more calories to stay fat enough. I would also always add Vitamin A.
If there is a book that explains all of these little differences and how to address them with the kind of hay or forage is at hand, I am not aware of it. But, whoever said you have to build your feeding program around your hay, had it right. Start out with your hay and then add what is necessary to complete the requirements. Be sure you get a horse to a 2:1 Ca:P ratio. We do that with a mineral that has 4 times as much Ca in it as P and also has a high level of Vitamin A. Then, I only grain the horses that need grain because they are working hard or need it to stay in good weight condition.
I know you will hear a lot to the contrary, but I have no problem feeding grain. I do not even think a little sweet feed will hurt any horse that needs weight and does not have a metabolic disorder (like PSSM, IR or HYPP or ???). I do not think grain products are evil. They must be fed carefully and not fed to horses with metabolic disorders. We have 50 horses and 49 of them can eat sweet feed or grain. We personally feed a grain based pellet instead of grain. I have a number of horses with bad teeth and that have a lot of age on them, so they digest and assimilate the grain pellets much better than they could chew and digest regular grain.