From amazon.com:

A novella from #1 New York Times bestselling author Brandon Sanderson, Legion is a fast-paced, witty, and supremely fun thriller with a psychological bent.

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.

His brain is getting a little crowded, however, and the aspects have a tendency of taking on lives of their own. When a company hires him to recover stolen property—a camera that can allegedly take pictures of the past—Stephen finds himself in an adventure crossing oceans and fighting terrorists. What he discovers may upend the foundation of three major world religions—and, perhaps, give him a vital clue into the true nature of his aspects.

From amazon.com:

Now also available in the complete collection Legion: The Many Lives of Stephen Leeds.

From #1 New York Times bestselling author Brandon Sanderson, Stephen Leeds is back in a double-length novella that Library Journal says has "the pulse of a thriller and the hook of a fascinating hero balancing on the edge of psychosis."

It's not his own genius that Stephen Leeds gets hired for. Clients want to tap into the imaginary experts that populate his mind—and it's getting a bit crowded in there.

Now Stephen and his internal team of "aspects" have been hired to track down a stolen corpse—but it's not the corpse that's important, it's what the corpse knows. The biotechnology company he worked for believes he encoded top-secret information in his DNA before he died, and if it falls into the wrong hands, that will mean disaster.

Meanwhile, Stephen's uneasy peace with his own hallucinations is beginning to fray at the edges, as he strives to understand how one of them could possibly have used Stephen's hand to shoot a real gun during the previous case. And some of those hallucinations think they know better than Stephen just how many aspects his mind should make room for. How long will he be able to hold himself together?

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.