The Sign of the Four by Sir Arthur Conan Doyle

Sign of the Four (1890) is the second novel featuring Sherlock Holmes written by Sir Arthur Conan Doyle. Doyle wrote four novels and 56 stories starring the fictional detective Sherlock Holmes.
The story The Sign of the Four is set in 1888. The Sign of the Four has a complex plot involving service in East India Company, India, the Indian Rebellion of 1857, a stolen treasure, and a secret pact among four convicts ("the Four" of the title) and two corrupt prison guards. The Sign of the Four presents the detective's drug habit and humanizes Sherlock Holmes.
A young lady named Mary Morstan comes to consult Sherlock Holmes upon a case which she assures him is very unusual. Miss Morstan's father disappeared some years previously under suspicious circumstances. He had obtained twelve months leave from his Indian regiment and sent word to her to meet him at his hotel. Whilst the hotel confirmed that he was staying there they informed Miss Morstan that he had gone out and not returned. Enquiries at the time produced no result. Major Sholto was Morstan's only known friend in London, the two had been in the same regiment together. Sholto had retired from service at that time and claimed not to have even known that Morstan was in the country.
At the outset of the novel The Sign of the Four Doctor Watson states that he has been watching Sherlock Holmes takes cocaine three times a day for many months. (Summary by Wikipedia)