(1935–2020) and Per Wahlöö (1926–1975) collaborated to produce a series of internationally acclaimed detective novels about the detective Martin Beck. Other writers followed. The most successful writer of Swedish detective novels is Henning Mankell (1948–2015), with his series on Kurt Wallander. They have been translated to 37 languages and have become bestsellers, particularly in Sweden and Germany. Mankell's detective stories have been widely praised for their sociological themes, examining the effects on a liberal culture of immigration, racism, neo-Nazism etc. Many of the stories have been filmed no less than three times, twice by Swedish companies and most recently in an English-language series starring Kenneth Branagh. But Mankell has also written several other acclaimed books, such as Comédia Infantil (1995), about an abandoned street boy in the city of Maputo. Several other Swedish detective writers have become popular abroad, particularly in Germany; for example Liza Marklund (1962–), Håkan Nesser (1950–), Åsa Larsson, Arne Dahl, Leif G. W. Persson, Johan Theorin, Camilla Läckberg, Mari Jungstedt and Åke Edwardson. From 2004 and onwards, the deceased Stieg Larsson caused an international sensation with the Millennium Trilogy, continuing as a series with new novels being written by David Lagercrantz. In the spy fiction genre, the most successful writer is Jan Guillou (1944–) and his best-selling books about the spy Carl Hamilton, many of which have also been filmed. Of Guillou's other works, the two most notable are his series on the Knight Templar Arn Magnusson and the semi-autobiographical novel with the metaphorical title Ondskan (The Evil). Ballads The Swedish ballad tradition had been initiated by Bellman in the late 18th century. In the 19th century, poetic songwriting fell into decline with the rise of university student choirs, until it was again revived in the 1890s. Poets increasingly continued the tradition of having their poetry set to music to give it a wider audience. In the early 1900s, a lot of poetry of the 90s poets Gustaf Fröding and Erik Axel Karlfeldt had been put to music. Arguably the most renowned Swedish troubadour of the 20th century was however Evert Taube (1890–1976). He established himself as a performing artist in 1920 and toured Sweden for about three decades. He is best known for songs about sailors, ballads about Argentina, and songs about the Swedish countryside. Between 1962 up until his death, the most highly regarded singer-songwriter in the Swedish ballad tradition was Dutch immigrant Cornelis