I use a book by Martin Evening - Adobe Photoshop 7.0 for photographers. This isnt light reading by any means, but does give explanations for why, not just how, things are done in a particular way. Like Peter, I have mainly just picked things up by trial and error, but that book is a good reference.
For what it's worth, my workflow in PS7 goes something like this (working with TIFFS derived from a RAW coverter or my slide scanner, but the principals should be similar for JPEG):
1 - crop, if necessary. This means any further adjustments will only be on data I actually want, speeding things up a little - at least with my very slow old computer. Also at this point I will remove dust spots and clone out unwanted bits.
2 - levels. Set white and black points, taking care to preserve highlight detail as much as possible.
3 - curves. Allows more subtle changes in tone response, brightness and contrast, as by changing the shape of different bits of the curve, you can affect different tonal ranges specifically
4 - if applicable, grayscale convesion, usually using the channel mixer
5 - resize as appropriate
6 - sharpen. I use unsharp mask mainly, though there are a lot of different ways of sharpening which I'm sure others will be better informed about.
When shooting RAW, a lot of the exposure management (steps 2 and 3) is done prior to conversion to TIFF, with only fine tuning done in PS
