This book is both a fashion joke and a must-have read for anyone who does retail shopping.
Read it for understanding how trends and trendlets are formed, for how clothes are conceived, manufactured, sold, marketed, and discounted. Read it for explanations of the differences between full retail, discount, outlet, and off-price stores and how and why to shop at each kind of store.
But absolutely take her actual fashion/style advice with a grain of salt. She lives a lifestyle very few people do, and, as evinced by the pictures of her on the back of the book and descriptions of some of her outfits, I think it's safe to say that she has a lot of money and very bad taste. (Her 'signature' look is lots of bangles and two wristwatches).
But she does know how the fashion industry works. So read it. Take what you can use and leave the rest.