If you really want to get into the science of soap making, Kevin Dunn' "Scientific Soapmaking" is highly recommended here. It's on my wish list!
Other than that one, though, I personally haven't found any soap making books I particularly like. They don't really go much into building your own recipes. They are mostly pretty pictures. They will have multiple different recipes but don't discuss the difference between them in anything besides "fluff" words. IMO, your money is pretty spend on molds and fragrance oils. My suggestion would be to hang out here for a while, get a few batches under your belt, and then look at soap making books to see if they offer what you want.