Doodling in your text books is bad! I do not recommend it. At all. However, let's say you have to doodle in your text—like, if you don't, you get in trouble. Then, by all means, please deface the book in the most entertaining way possible.
Above, you can see Chinese revolutionary Sun Yat-sen getting the Mario treatment in a Japanese textbook. And below, you can see schoolbooks from throughout Asia that have been defaced in a variety of creative ways. A few of them you might have seen before. But all of them, amazingly, are cock-free. That's right, you don't need to doodle dicks to be entertaining when you deface your books. It doesn't hurt, though.