The Second Edition of Real World Image Sharpening with Adobe Photoshop, Camera Raw, and Lightroom has just been published (U.K. link). Originally written by the late Bruce Fraser, the book has been updated by Jeff Schewe. It remains of the few "must have" technical books for digital photographers...all digital photographers, whether you work with an iPhone or a medium-format digital back. You might not think you need a whole book on sharpening, but take my word for it—you do. If you don't have this yet, do not hesitate, "buy it now." Winter's coming*, and you'll need something to read.
*In the Northern Hemisphere. If you live Down Under, summer's coming, then winter's coming.
Featured Comment by Moose: "I would go further and say this book is not just for 'all digital photographers, whether you work with an iPhone or a medium-format digital back,' but for all those who work with any images that enter a digital form, including those that are scanned from film or prints. I suppose nothing is indispensable, but this book is as close as possible. I found the first edition revelatory, both in explaining why some things I had discovered worked and introducing me to other techniques."