If you are an aspiring caricaturist, or even if you are a professional, you need to buy this book, written by MAD artist extraordinaire Tom Richmond:
I recently had the pleasure of being a backup, proofreading pair of eyeballs to the real (and amazing) copyeditor of this book, my friend Celestia Ward. So, I am not just banking on Tom's reputation as an incredibly hard-working artist and entertaining (and prolific) blog author---I've read it closely from cover to cover, and it contains:
1. concise explanations and demonstrations of concepts I've never considered that are relevant to caricature
2. things that I had considered, but just never articulated in my head (i.e, things I have always done but never thought about WHY).
3. a treatise on every facial feature
3. Numerous illustrations to make everything clear visually
4. a few in-jokes and familiar faces used as samples
Here's a link to Tom's blog showing a few pages: Head shapes
If you buy now, the first 100 orders get a hand-drawn piece of art from Tom!
My own 'hand-drawn piece of art' by Tom is a caricature he did of me (back when I had brown hair, alas) at my second ISCA con in 2004. It's a pen-and-ink wash, which thrilled me because I love that medium:
For that, we'd have to go back to 1991, when I bought an unusual comic book (and they were not my thing, I was never into super-heroes). Actually, I bought it as a Christmas gift for my then-husband, but of course, I read it cover to cover myself, because it was about a guy as far from a super hero as you can get: