I'm looking for an answer to the same question I did also hear that the Pippa Funnell was for the budding eventer, and unless that's what you want to do don't bother.
Try Anthony Crossley's Training the Young Horse and Reiner Klimke's Basic Training of the Young Horse. Both very down to earth and very easy to understand the logical progression of riding youngsters..
I agree the Pippa Funnell one is good and suitable for any young horse not just eventers. But my favourite is the Reiner Klimke one - Basic Training of the Young Horse... it is great!
All the above and Sylvia Loch's Dressage In Lightness which is an excellent book as the photos really do explain a lot of the typical issues schooling youngsters but obviously flat work only. Very good on the progression / stages much as the Reiner Klimke book.