In which novel do the 'Baker Street Irregulars' first appear?

The Baker Street Irregulars are fictional characters who appear in various Sherlock Holmes stories, as street children who are employed by Holmes as intelligence agents. The name has subsequently been adopted by other organizations, most notably a prestigious and exclusive literary society founded in the United States by Christopher Morley in 1934.

The group of street urchins is led by an older boy called Wiggins, whom Holmes paid a shilling per day (plus expenses), with a guinea prize (worth one pound and one shilling) for a vital clue, to collect data for his investigations.

The group appears in the first Sherlock Holmes story, A Study In Scarlet (1887). They also appear in the next novel, The Sign of the Four (1890), in which one of the chapters is titled "The Baker Street Irregulars".