The Permaculture A Designers' Manual by Bill Mollison is the most in-depth resource in permaculture design, and best for those deeply committed to the implementation and understanding of the movement. Join me as I explain every chapter briefly.
While the book is in-depth, it offers everything a designer needs. It's not an easy read cover-to-cover, but more of an essential manual to understand the workings of each part of permaculture and how they work together to make the concept function properly in your area.
The Permaculture Designers Manual (Available in print only)