Principles from Permaculture a Designers' Manual by Bill Mollison.

This is the definitive Permaculture design manual in print since 1988. It is the textbook and curriculum for the 72-hour Certificate course in Permaculture Design.

 

You may also be interested in this podcast, where Geoff Lawton covers the contents of the Designers' Manual in about an hour.

Permaculture: A Designers' Manual - Bill Mollison