It says that when a plate edge slides with another plate edge it causes a vertical fault, as San Andreas fault. I thought that SAF is a horizontal fault i.e. strike-slip fault. So what does the book mean by vertical faults?
See the image below. In the picture, the San Andreas Fault dip is nearly 90 degrees or exactly vertical; The strike is pointing North.
Most normal faults and reverse faults (such as the Sierra Madre) are not vertical, but rather dip in either direction. (In the picture the Sierra Madre appears to have a dip of about 45 degrees).
In real life, the dip of the San Andreas is not perfectly vertical, but is variable, depending on where in California you are.
Strike and dip can be measured using a Brutton Compass.