How did the Nancy Drew series get started? It was originally released as a trilogy of books in 1930, along "The Chester Boys" and "Slim Tyler Air Stories".
Elmer Dawson's "The Chester Boys", or "Buck and Larry Baseball Stories" were mildly popular; they got a sequel in 1931, and another sequel in 1932, before the series ended.
Richard Stone's "Slim Tyler Air Stories" did a little better; it got two sequels in 1931, and a third sequel in 1932, before the series was cancelled. Wikipedia says the series had a seventh entry in 1935, but all of the antique book websites I've seen deny its existence.
Nancy Drew, on the other hand, was immediately popular. They wrote a fourth book as soon as possible, and it became a regular series.