I purchased "Smart Soapmaking" by Anne L. Watson and I do like this book. If you go to Amazon.com you can look up books there and read reviews written by soapers who have read the books.
Honestly, I have to recommend borrowing all of them from your library first, lol. I have looked on amazon for ones I want, then borrowed from library, and was so grateful I didn't pay for it.
My suggestion is to read reviews (amazon.com) then compare the prices for the same books on half.com. More times than not they are cheaper on half.com. Also for many, like myself, the library is not an option. Shoot, in our local library there is only one little section, about 4 shelves at 3 ft. long each with CRAFT books. In there...there are NO soapmaking books!
I agree about the library, that is where i found all of the books that I later bought. If you are interested in Hot Process a great book is Handcrafted Soap by Delores Boone. I still use some of her recipes in CP. Also The Everything Soapmaking book by Alicia Grosso and lastly Soapmaker's Companion by Susan Cavitch.