Multiplicity : the new science of personality, identity, and the self / Rita Carter.

Enhanced descriptions from Syndetics:

Multiplicity presents an entirely new view of our selves. Instead of seeing each person as a single personality, Carter argues that we all consist of multiple characters, each one with its own viewpoint, emotions and ambitions. The mother who feeds breakfast to her children, for example, has quite different concerns and opinions from the woman taking part in a boardroom discussion two hours later, and from the woman she will be with her husband that night. Yet all three may share the same body, and none is any more "authentic" than another.

Personality changes in a person are conventionally frowned upon, but Carter shows that in today's world our ability to switch from one personality to another according to what is demanded of us is a huge strength, providing one's personalities work together as a team rather than against each other. In addition to its groundbreaking scientific thesis, Multiplicity contains extensive exercises designed to help readers achieve this harmony.