I did a book review of
Learn to Draw with Anime Studio by Philippe Cizaire (Hypercomics):
http://generalpicture.com/animation/?p=48
where I argue that Cizaire's lovely book should not be passed off as for kids only.
More guidance on drawing characters (again, more about drawing than about the essence of characters) comes from the impeccable tutorial by Dr. Nick:
viewtopic.php?t=8904
(Dr. Nick's work is also an object lesson in how teach your own skills to other people.)
But I think you want to learn more about character design than drawing your designs.
OK, there are plenty of the fundamental tricks in character design to be found in Tom Bancroft's
Creating Characters with Personality. Watson-Guptill, 2006. Unfortunately, the personalities favored by Bancroft have all the flavor of lukewarm dishwater--yuk! We've seen his characters before from animation studios of the 1960s, where they were invariably better conceived. Try to get beyond that when you read his book and you can learn lots from him. I would also add that Bancroft's whole mindset is very mainstream/Hollywood/mass market/cereal-selling, but this is not the only way to think about characters!
If you want to see the kind of heart and soul which is entirely lacking from Bancroft's Mighty Morphin Sugar Doodles idea of art, feast on the brilliant character design in Tony White's
The Animator's Workbook. Watson-Guptill, 1986. Ironically, White's book isn't specifically billed as relevant to character design.