Complex sentence: Stratemeyer had created the Hardy Boys series in 1926 (although the first volumes were not published until 1927), which had been such a success that he decided on a similar series for girls, featuring an amateur girl detective as the heroine.
Split into simpler sentences: Stratemeyer had created the Hardy Boys series in 1926 (although the first volumes were not published until 1927). The series had been such a success that he decided to create a similar series for girls, with an amateur girl detective as the heroine.