What is the Crockford that Sherlock Holmes refers to in 'The Adventure of the Retired Colourman' by Sir Arthur Conan Doyle?

In "The Adventure of the Retired Colourman," Sherlock Holmes refers to Crockford as a well-known directory of the British upper class, specifically detailing the members of the clergy and their respective connections. It serves as a resource for identifying individuals and their social standings, which Holmes uses to investigate the case. The mention of Crockford highlights Holmes's reliance on records and social networks to gather information relevant to his investigations.