""...for all that the murder and the subsequent events did actually take place, this reads like a riveting work of fiction.""
On November 11 1856, the cashier of the Midlands Great Western Railway, George Little, was murdered in his office at the Broadstone Terminus in Dublin. He received an injury to his head, and his throat had been cut. The thing that baffled the police was that his office was locked from the inside. The murder was therefore a real live locked-room mystery. Only some of the great sums of money in the room had been left untouched, so it wasn't a robbery. So what was the motive, and more importantly, who did it?
A huge investigation got underway. Dublin's leading detective and leading lawyer came together to solve the case. But they were baffled. So too were two detectives sent from the Metropolitan police force in London, who give up after a fortnight. The case dragged on for months, eliciting much criticism in the newspapers. Then there was a crucial breakthrough, though not because of the police investigation. A woman came forward and said that she thought her husband did it. Detectives investigated, and eventually built up a case based mainly on circumstantial evidence. The man (I won't do a spoiler by giving his name away here) was brought to trial, and to the dismay of the detectives, and to the people of Ireland and England, was found not guilty.
But the man's troubles didn't end there. He came to England, and was examined in Liverpool by an English phrenologist, who was convinced he could tell a person's murderous tendencies by measuring their skull. At the same time, the man also wanted to emigrate to the USA, as he was reviled by Irish and English people alike, but had no money. Eventually, he emigrated, along with his son, and little more was heard of him.
As to how the murder was committed while the door was locked from the inside - well, you'll have to read the book to find out.