Explanation: A pager or a beeper is a small electronic device (not used much anymore because so many people have cell phones) that beeps or vibrates and shows the telephone number of the caller or a message from the caller. They are often used in hospitals or other locations where it is important to contact someone and do so with minimal interference.
This joke is funny because books have pages but do not have pagers. You can also reach a book with your arm. Read a good book today!