I can't help with a recommendation but you'd need to know about good basic horse management (so you can pick up on those neglecting their horses and so you can offer horse owners what they actually need instead of unsuitable conditions), grassland management (so your fields aren't ruined), info on running a business (so many yard owners are unhappy because basically their yards are badly run - which is down to them!) including laws and staff etc. So maybe more than one book.
