And the Truth by Lester del Rey

Journalist Dane Phillips has spent a decade chasing a shocking truth: aliens live among us, able to regenerate from fatal wounds and impersonate humans perfectly. Repeatedly dismissed and ridiculed, Dane’s obsession grows desperate after discovering another alien masquerading as Milo Blanding, a recent accident victim.

Determined to uncover the truth, Dane plans to exhume Blanding’s grave. However, his investigation is thwarted by a sinister conspiracy involving his own wife and psychiatrist. Trapped and facing mind-erasing "therapy," Dane's final act will reveal a terrifying truth about himself, pushing him to the brink of sanity...