"Skin" is dreadful. I glanced through it and quickly decided I feel sorry for anyone who buys it. Katrin Eismann is a competent instructor but I think her beauty work is heavy handed and the images she uses as samples don't inspire.
Photoshop is an artist's tool and no two artists view art the same way, especially in the realm of cosmetic beauty. The genius book on skin has not been written yet and I sincerely doubt a good one can come from the inner circle of the current "in" crowd of gurus because you need to be an expert at beauty/cosmetic imaging. I wouldn't trust any skin book if it doesn't offer several supermodels and high-end cosmetic or dermatology ads.
Art is often about taste and high-end advertising demands the best from pro retouchers. Whether the look required is "no pore" or "open pore," skin retouching has to be perfect, and the truly great retouching work is costly and time consuming. It also helps if your skin retouchers are make-up artists. Studying the art of make-up can help beauty photographers achieve better results.
I like Martin Evening. Among the "in" crowd of Photoshop gurus, he understands cosmetic beauty but his scope lacks breadth. His strength is the "no pore" look, even though he insists he's left visible pores, but I've yet to see what he can do with a make-up-less face shot at F8 with a large format camera. Retouching natural, clean skin, without blurring its pores, requires much patience and attention to detail, and it requires a tool-set formula. Unless you're a professional make-up artist, most photographers give-up after the first five minutes and fall prey to the mush-producing Healing Brush.