Which book gives the best advice on teaching a youngster to ride.?

My son aged 4 is just starting to ride. He cannot have lessons at the local riding school as they dont take them until aged 5. So I want to start him gently myself. I am an experienced rider but dont want to go at it like a bull in a china shop. Can anyone suggest a good book that is aimed solely at teaching a youngster? Or do you know of a riding school that take them at 4 in the Warwickshire (UK) area? Thank you (-: