There are two Sharpening must-reads - first, to lay the groundwork, is Thom Hogan's once over lightly
Sharpening 101.
The second, much more useful and detailed is in Ron Bigelow's
superb series on photography. The articles on sharpening are in the Editing section and the first one is at
this link.
A subject that is covered only vaguely in Hogan's article and not at all, I think, by Bigelow is
Local Contrast Enhancement (LCE). LCE is an amzing technique to increase the micro-contrast in areas on an image and is the concept behind the clarity slider in LR.
One last comment. It isn't always necessary to sharpen an entire image. For example, if you are spending a lot of time and effort to make the foreground object prominent and defocus the background, why then sharpen the background? A typical use of selective sharpeningis in portraits where a lovely defocused
bokeh is desirable. A way to do this is to bring the subject up onto another layer either through selection or a mask and then sharpen only the subject.