Is there any book anywhere that couldn't be in need of a trigger warning? Think about Twilight or Lord of the Rings or The Great Gatsby. What would be helpful is for professors to do what many I know already do: ask students at the beginning of a class to inform them privately if they have any issues that might interfere with classroom learning and proceed from there.
Once you set the precedent that speech will be censored if it offends people who respond violently to that speech, you guarantee that more people will respond violently on a wider variety of issues. There is a reason why no one ever says "the best way to deal with a bully is to immediately give him everything he wants."