From amazon.com:
The shocking conclusion to Brandon Sanderson's Legion novella series, on its own for the first time.
Stephen Leeds is perfectly sane. It’s his hallucinations who are mad.
A genius of unrivaled aptitude, Stephen can learn any new skill, vocation, or art in a matter of hours. However, to contain all of this, his mind creates hallucinatory people—Stephen calls them aspects—to hold and manifest the information. Wherever he goes, he is joined by a team of imaginary experts to give advice, interpretation, and explanation. He uses them to solve problems . . . for a price.
When Stephen gets an unexpected cry for help from the woman who taught him how to control his aspects—right when one of his aspects suddenly and violently disappears—it will send him on a case the likes of which he could never have imagined.