as far as i know there is no book special herbs for horses.
But there are many for humans. If you buy that one and take a list of toxic herbs for horses then you know what you may grow in your field. So they can choose by themselves.
If you wanna give horses some herbs you have to know exactly what you are doing. Sometimes can a combination give not the result you are looking for.
If they can choose bij them selves is better, but you need a big part of your field and take care of the plants.
that will cost a lot of time
Also each herb needs a different place to grow, soil, shadow and so on. So you need some background info if you wanted to buy herbs to put in in your field.