The APA Standard is definitely worth buying if you can afford it. The breed-specific information is only part of the book. The other portion is information that applies to all birds, and reading through it can be quite useful to the new breeder in assessing the "whole bird," not just a specific part of one breed. But until you can get a copy, here is a place online where the Dorking specific portion of the Standard is available at no charge:
http://dorkingbreedersclub.webs.com/us-dorking-standard-colors