As a verb (American):

To depart, leave. A fashionable term of the 1990s in black street usage and also heard among white adolescents. A variety of euphemisms (like its contemporaries bail, bill, jam and jet) for 'run away' are essential to the argot of gang members and their playground imitators. The origin of this usage is not certain; it may derive from an earlier phrase 'book it', meaning that someone has to return home quickly in order to record a trans- action.

As an adjective (British):

Cool. When using predictive text in text messaging, an attempt to enter 'cool' will prompt the option 'book', hence its ironic substitution by teenagers also in speech.