I totally agree, and have both of those books myself. They are a treasure trove of information regarding safety, usage rates, and constituents. Constituents are to EOs what fatty acids are to oils/butters; knowing which constituents are in which EOs can help you decide which to use, and why. Of course, no one knows which EO constituents survive the lye monster, but for leave-on products, this information is really important, IMO.Essential oils books by tisserand have loads of info about essential oils, usage rates, sensitivities etc.
Another book I would recommend, IMO perfect for beginners or those looking for more knowledge, is Worwood: the complete book of essential oils and aromatherapy . I've given it as gifts and have heard only good things from those who use and reference this book. It's one of my faves.