If you have a free hour, some colored pencils, and a little artist buried inside you, then grab a copy of Doodletopia Manga.
I've always admired the manga characters that have been appearing all over lately. There's manga-illustrated tv series and comic books, manga style graphic novels, and manga on t-shirts here and there. Now, the question is how do you sketch those not-quite-cartoon characters?
Doodletopia Manga gives you an introduction. You can sketch along right in the workbook with the provided starter images, and then you can take your wildest manga imagination and set it free on your own paper.
There are plenty of sketching exercises here, from capturing lively character expressions to clothing your manga men and women. This book would be perfect for a middle-school age or older artist.
Fun times. :-)