I second what Kevin said. I have Jason Seiler's caricature dvd and it's well worth getting if you really want to learn about how to do it, he uses all sorts of mediums including digital and talks about how to read a face and see the weight and other areas that you can exaggerate or play down.
Kruger does great work too, his more recent stuff is huge and photorealistic, his books can be expensive though. Derren Brown does work that's very similar to Kruger and his book is far cheaper, I have a copy and as Kevin said they're not teaching books but are great references to see how someone else does it.
Derren Brown book: