As for clothes: Pick up a copy of David Kibbe's "Metamorphosis". Learn what clothing shapes, patterns (stripes, polka dots, etc) and colors work best for you. Get a good hair cut for your hair type and face shape. If you have any health issues, work on those. Healthy people look better, regardless of what they wear. Once you have some idea of the shapes, colors, etc you need to focus on, try on lots and lots of clothes but only buy those things that are both very comfortable to wear and are also flattering. (With practice, it gets easier to shop once you know what works for you. You can eliminate most clothes in the store without trying them on once you have this knowledge.) If you are physically comfortable in your clothes, it helps you pull off a sense of "style" in the eyes of others. Elegance is not just about the clothes you wear but about how you move and that is enhanced by being physically comfortable. If you have self image issues negatively impacting your clothing choices, the best solution to that is probably having a positive intimate relationship with the right person so you Get Over It.
If you want to learn about power dressing, there is only one book that I know of which is based on actual research into what works: "Dress for Success" by John T. Malloy. It's apparently been updated a few times. Searching for it on Amazon brings up "New Women's Dress for Success".
I've read a lot of clothing books over the years. A lot of them don't have much value. If you want good results without investing a lot of time and effort into it, those two books are must haves and most of the rest can be ignored.