For a beginner, I think the Scott Kelby books are probably easiest to understand, and if you don't mind his corny sense of humour, they are well written and organised.
The Adobe Photoshop Lightroom 4 Book for Digital Photographers
The Adobe Photoshop CS6 Book for Digital Photographers
Since you specifically mentioned portraits, his Professional Portrait Retouching Techniques for Photographers Using Photoshop is very good. Does not really teach you post-processing basics, more along the line of skin softening, eyes/teeth brightening and so forth. If you are a beginner you'd want something to familiarise yourself with the basics first I think, although his techniques are easy to follow.
I also recommend his 7-Point System for Adobe Photoshop CS3. This takes you through many different images from start to finish. You can download all of the images used in the book, so you can work the same images yourself.
For more in-depth books I'd suggest something by Jeff Schewe (RAW processing)
The Digital Negative: Raw Image Processing in Lightroom, Camera Raw, and Photoshop
or Martin Evening (for Photoshop)
Adobe Photoshop CS5 for Photographers: A professional image editor's guide to the creative use of Photoshop for the Macintosh and PC
Finally, lynda.com and kelbytraining.com have very good, in-depth online tutorials. These can be several hours long each. Cost is around $29/month, and for many people I think these would be easier to learn from than books.