The Valley of Fear | Arthur Conan Doyle | Part 1 - Chapter 1 (Bite 2)

The man who would become the most famous detective in the world sat with an untouched breakfast, staring at the ceiling and working through a cipher by pure reason. The message before him was a jumble of numbers and letters, but Holmes saw in it the outline of a book. The first clue was "534" — almost certainly a page number. From there, he deduced the volume was large, printed in double columns, and common enough that both sender and receiver would own identical copies. The Bible was too variable in pagination; Bradshaw's railway guide was too limited in vocabulary. The answer was an almanac. But the new year's almanac failed him — the words it produced were nonsense. Then Holmes realized the error: the message had been written in December, using the old year's edition. When he switched to the previous year's almanac, the cipher unlocked. "Danger," it read. "May come very soon. One. Douglas. Rich country. Now at Burlstone House. Burlstone. Confidence is pressing."

Holmes was still savoring the elegance of his solution when Inspector Alec MacDonald of Scotland Yard arrived. MacDonald was a young, capable detective who had learned to respect Holmes's genius. He had come with grim news: Mr. Douglas of Burlstone Manor House had been murdered the night before. The cipher had predicted the crime before the detective knew it had occurred.

The logic of the cipher