This book started it all. Watson needs to find a cheaper place to live. Holmes is looking for a roommate. They move to 221B and the legend is born. The first use of a magnifying glass by a detective is here too.
The mystery: Scotland Yard asks Sherlock Holmes for help investigating a murder. Holmes has a look around and says the murderer is 6 feet tall, has long fingernails on his right hand, smokes cigars, was taken to the scene in a carriage pulled by a horse with 3 old shoes and 1 new one, and the man was poisoned. Wow! But that's Sherlock.
Arthur Conan Doyle was criticized for his depiction of Mormons in this book. He eventually apologized. And Holmes kills a dog to find out if a pill contained poison. It did. I didn't much like that scene but that was the 1880s and sensibilities were different then. Check out the classic mystery A Study in Scarlet (1887).