not a book, but (a) free (b) pitched right for anyone who knows a bit about both photgraphy and post-processing but wants to go next stage deeper
http://www.cambridgeincolour.com/tutorials.htm
by an obviously switched on photographer and also including difference in light perception by camera and human and, although assuming photoshop, I still find easy to follow for other progs or Elements 4 + plug-in; nice use of befores/afters using mouse positionioning. Have saved some of the key pages for when away from connection.