There is a book called bonsai styles of the world. I forget who the author is, but I believe you can get it from, stonelantern.com. The author is from south Africa and there is a huge chapter on the "pierneef style" like this acacia and other African styles. It is an original and informative book on modern styles and forms of bonsai. reading and studying books like that and of cource robert steven's books will realy boost your creativity.