The one I want to have a look at is Alison Pocklingtons care of the event horse - she was BHS groom of the year and worked for Chris Bartle. I was hoping it might be useful as the Manual of Horsemanship is very droll!
I also have The Ultimate Book of the Horse and Rider by Judith Draper, Debby Sly, and Sarah Muir. It covers everything from breeds of the world, riding, tack, veterinary, and grooming. Although it is a tad bit outdated, it is a very good book!
One I found in my bookcase last night was Handling Horses my General someone or other. First published in 1943 and reprinted in 1945. I could not put it down it was so accurate and apart from being a bit dated most of the principles are the same. Sorry doesnt help you in choosing but thought it was amusing.