What would be the best book for FASHION DESIGN THEORY/PRINCIPLES?

I am very into fashion design and have taught myself about it by reading books. I have designed many collections (sketches) since the start of 2009 and have gone through quite of lot of fashion books about concepts/illustration/history/constructi… but now I am looking for a book which actually teaches you about the design principles and theories. I really want to learn, it feels horrible when I compare my designs to Karl Lagerfeld's and Valentino's (i know i'm only 14 but i have a great ambition!). I've tried finding it on google but it was a pain. It would be much appreciated if you can recommend a title for me, thank you!