The Soapmaker's Companion by Susan Miller Cavitch A great book for those who need the why as well as the how. Very comprehensive.
The Art of Soapmaking by Merilyn Mohr Lovingly written with interesting background information, good for those wanting to use animal fats in their soap, possibly a little old fashioned.
The Complete Soapmaker by Norma Coney Not really complete, nor entirely helpful this is the book I started and struggled with. However, its one of the few to give great instructions for re-batching. Not something many soapmakers like to fiddle with, re-batching can save a bum batch or allow you to incorporate delicate and/or expensive ingredients without exposing them to the original, caustic batch.
Soap: Making It, Enjoying it by Ann Bramson I love the historical informaion in this book and many find the book itself somewhat historical. Yes, soapmaking has come a long way and the low temperatures she recommends threw me off track for awhile, BUT, the basics are there and I love it anyway.
Milk Soapmaking by Anne L. Watson Perfect for anyone interested in milk soapmaking, even those new to soapmaking.
On the web:
http://www.annelwatson.com/soapmaking A my newest favorite soapmaking spot on the web.
http://www.millersoap.com A favorite spot on the web for no nonsense soapmaking basics and troubleshooting.
http://mountainroseherbs.com/learn/essential_oils_handle_with_care.html A great guide to essential oil safety.
http://www.thedishforum.com Do we have forums? Yes, we do! You'll have to register to access this chat and adjoinging swap site, but its a hotbed of helpful conversations concerning all aspects of making and selling all types of soap and toiletries, all natural or not.
The Handcrafted Soapmakers Guild This non-profit trade association offers a variety of resources for the professional and hobbyist soapmaker as well as the handmade soap connoisseur. This link will take you to myth disspelling descriptions of the various types of handmade soap and processes. Benefits of membership run the gamut from education to liability insurance, web hosting and annual conferences. Based in Portland, OR!