In my searching, I found three books to be extremely helpful that now go with me just about everywhere these days - they're that good.
This is fantastically more than just a simple how-to book on character design. It's largely geared towards characters for kids, but the underlying principles really span the entire board.
"The Mad Art of Caricature" by Tom Richmond (of MAD magazine fame)
This is an even more in-depth look at pulling the personality of a character out through the features of a person. Still working through this one - it's chock-full of great insight.
This one is pretty straight-forward. It has an absolute TON of facial expressions from different people - young and old, guys and gals. A lot of diversity between the facial features.
All of these books are pretty cheap, so there's no excuse not to have them in your library if you're interested in character design at all!
Anybody have any other great recommendations to my ever-expanding illustration resource list?
Now to see if I can get some sort of commission for this little sales pitch... :)