This chapter contains a lot of details, but the overall message is that, when it comes to sharpening, one size doesn't fit all. If you fail to take the image source into account, you end up sharpening noise as well as detail. If you ignore the image content, you may exaggerate unwanted detail, or obscure wanted detail. And if you don't tailor the sharpening for the output process, your images will appear undersharpened or oversharpened.
The challenge, then, is to reconcile these disparate needs to produce optimally sharpened images. In the next chapter, Sharpening Strategies, I'll discuss a sharpening workflow that attempts to do just that.