Here are a few options for what Viet might be doing with his book:

• writing - This is the most likely option, as people often write in books.
• drawing - If Viet is artistic, he could be drawing in his book.
• reading - He could be reading something in his book.
• sketching - He might be making quick drawings or notes in his book.
• doodling - If he's bored, he could be doodling in his book.