A book kept on a tilted writing desk starts sliding then the direction of frictional force is __________ to the direction of motion.

The force that opposes the movement of a solid object over another is called friction. Friction is categorised into static, sliding, rolling, and fluid friction. Friction and normal force are proportional to the contact surfaces and independent of the hardness of the contacting surface. 

For example, a frictional force is created when a book is placed on the writing desk. As the object moves, the frictional force, called sliding friction, operates in the opposite direction.
Sliding friction is the resistance created between any two objects when both are sliding against each other. For example, sliding the block across the floor and two cards sliding against each other in a deck exemplify sliding friction.