The "Book test" (or "Book sign") is a simple part of the physical exam used to confirm the diagnosis of lateral epicondylitis, or tennis elbow. The basic problem in this condition is inflammation of the tendon complex, muscles, and bone around the lateral epicondyle - the knobby part on the outside of the elbow. The tendon of wrist extensor muscles (the muscles that pull the hand back) has its origin on that bony prominence.
The book test, along with palpating (mashing on) the tendon complex, is a good way to confirm tennis elbow, and to differentiate it from other conditions that can cause elbow pain. To perform the test, simply get a hefty book (I use the PDR that is in each room), and have the sufferer put his or her hand out palm up. Gently rest the book on their outstretched palm. There should not be significant discomfort even with tennis elbow present. Then, hold the book a bit lower and have them turn their hand palm down try to pick it up from above. If tennis elbow is present, this will hurt - usually a lot - right over the inflamed tendon.