Dr Sheppard's manuscript

In the novel The Murder of Roger Ackroyd, Dr Sheppard wrote a manuscript about his friend Roger Ackroyd's murder. It started with Mrs Ferrars' suicide. The Belgian detective Hercule Poirot remarked that there were many blank spaces in the manuscript. He said that they were drastically different from the manuscripts of Arthur Hastings, where "he always talked about what  he  thought, what  he  did--his name was written everywhere".

Spoilers ahead!
When Poirot exposes Dr Sheppard as the murderer (and thus Agatha Christie's first unreliable narrator), he asks Sheppard to complete the manuscript, and later silently take his own life. He follows Poirot's advice and takes an overdose of barbiturates.

In adaptations
In Agatha Christie's Poirot, the manuscript is turned into a journal; and Dr Sheppard's voice was over the events of the novel. Sheppard's spinster sister Caroline was curious about the memories and thus realized that James was a killer and a blackmailer. In the original novel, Caroline didn't know anything and thus her brother killed himself to avoid that Miss Sheppard knew the truth.