I'm differentiating between identifying with a character and being motivated by that to take a political action and feeling part of a societal group. Based on my personal experience, I think that books can definitely do the former but don't do the latter, whereas television can. If anything, I think that the example of cult movies supports that idea. Huge groups connect with them, whereas even the most popular books generally only inspire a small elite to break them apart, take them out of context, etc.