my college books say that it is a myth:
One of the most popular myths in psychology is that we use only 10 percent of our brain. This myth might have begun with early research showing that approximately three-fourths of the cortex is “uncommitted” (with no precise, specific function responsive to electrical brain stimulation). These areas are not dormant, however. They are clearly involved in interpreting, integrating, and acting on information processed by other parts of the brain. They are called association areas because they associate, or connect, various areas and functions of the brain. The association areas in the frontal lobe, for example, help in decision making and planning. Similarly, the association area right in front of the motor cortex is involved in the planning of voluntary movement. (Carpenter 54-55)
Carpenter, Siri. Visualizing Psychology, 2nd Edition. John Wiley & Sons, 2009.