Principles of Ecological Design, Ludwig, Art, ©1989 Oasis Design, oasisdesign.net. This short manual gives rules for ecological design that are both simple and profound. An excellent companion to his other books which deal with greywater and water storage.
Gaia’s Garden, Hemenway, Tony, © 2000 Chelsea Green. This is the most accessible book about Permaculture for gardeners, especially for the West Coast.
Permaculture: Principles and Pathways Beyond Sustainability, Holmgren, David, © 2002, Holmgren Design Services. A more theoretical approach to sustainable design concepts.
Permaculture: A Designers' Manual, Mollison, Bill, © 1988 Tagari Publications. This comprehensive book is the textbook for the Permaculture Design class. A reference for those who have already been introduced to the principles, as well as a dual duty doorstop and blunt instrument :-). It is on this list for the sake of comprehensiveness.
Design with Nature, McHarg, Ian, © 1982 John Wiley and Sons, Inc. This book is more a lanscape-level and regional-planning book, but has beautiful graphics and exposition about where to site projects. This book addresses the too-rarely asked question, “where is this project?” when designing.
The Power of Limits: Proportional Harmonies in Nature, Art and Architecture, Gyorgy Doczi, © 1981 Shambhala Publications, Inc. How the Fibonacci sequence and other aspects of the golden mean underlie proportions in nature, and how this has been used historically in good design.