the Sign of Four by Sir Arthur Conan Doyle (1890)
Sherlock Holmes and Dr John Watson are getting used to another although Dr Watson is concerned about his friend's drug habits; cocaine or morphine depending on the day.
A young woman is looking for Holmes' advice concerning a strange case. Although her father is missing, she is receiving a pearl once a year and a letter that claims to know the whereabouts of her father. She asks Holmes and Dr Watson to join her and they are eventually led to the house of a Mr Shonto. But they find Mr Shonto dead inside a locked room and Sherlock has to solve the case.
The second Sherlock Holmes book and I did not really enjoy it. The main reason is that it is too short and too rushed. I'm sure the next one will be much better.