Why do job interviewers ask what you read, and what's the right answer?

The interviewer believes you are what you read. Generally, stick to non-fiction. If you're in a recognizable field, such as finance, visit a bookstore and determine the new books on the subject. But don't claim to have read a book you haven't cracked open because the interviewer may be familiar with the work and catch you in a false statement.
Broader answers include: daily newspapers, historical books, technical magazines and biographies.
Be discriminating about your choice of biographies - the life of a serial killer, for instance, could raise flags. Choose non-controversial or ancient biographical subjects when an interviewer uses the reading question as a window to your mind.