"The Sign of the Four" by Arthur Conan Doyle is a detective novel published in 1890. When Mary Morstan seeks Sherlock Holmes's help regarding her missing father and mysterious annual gifts of pearls, the investigation leads to a buried treasure, a vengeful one-legged man, and murder. Holmes and Watson must pursue suspects through London, unraveling a decades-old conspiracy that stretches from India's colonial past to the Thames. The case tests both Holmes's deductive powers and Watson's loyalty to their intriguing client.
Wikipedia page about this book: en.wikipedia.org/wiki/The_Sign_of_the_Four