Facial Expressions: Babies To Teens; A Visual Reference For Artists by Mark Simon is probably the most unique and possibly the most valuable. It's a big book, 256 pages (with a free Internet supplement available), and entirely filled with reference pictures of, you got it, babies to teens. Each model tends to get between 2 to 4 pages of 2 1/4 inch to 2 1/2 inch square head shots with a tremendous variety of expressions and poses. Other chapters include a skull gallery, hats and headgear, a phonemes gallery (mouth shapes for various sounds), and an age-progression gallery (shots of the same model over a wide range of years).
Well that's it actually. It's a big book of photos, well put together and incredibly handy to have in reach while at the drawing table. I recommend this book to any artist involved with character design or illustration.
pictures taken from Facial Expression Babies to Teens all images are copyrighted