Well there are a couple of books you could look at:
New Sensations for Horse and Rider: Introducing Voice Training by Tanya Larrigan and The Young Horse: Breaking and Training by Jennie Loriston-Clarke
And some videos to watch, you can find details at:
http://www.horse-videos.com/The British horse Society has a web site with information on training:
http://www.bhs.org.uk/