The Secret Techniques of Bonsai by Masakuni Kawasumi II with Masakuni Kawasumi III is predominantly in black and white which was a bit of a disappointment, but seems to go very deep into most techniques i've heard of an many that I haven't.
The other is Bonsai Secrets by Peter Chan, which seems a bit more accessible, has beautiful illustrations and shows a great variety of old, gorgeous trees...
I'll write a bit more once I've had the chance to read these properly...
