Swedish literature refers to literature written in the Swedish language or by writers from Sweden. The first literary text from Sweden is the Rök Runestone, carved during the Viking Age circa 800 AD. With the conversion of the land to Christianity around 1100 AD, Sweden entered the Middle Ages, during which monastic writers preferred to use Latin. Therefore there are only a few texts in the Old Swedish from that period. Swedish literature only flourished when the Swedish language was standardized in the 16th century, a standardization largely due to the full translation of the Bible into Swedish in 1541. This translation is the so-called Gustav Vasa Bible. With improved education and the freedom brought by secularisation, the 17th century saw several notable authors develop the Swedish language further. Some key figures include Georg Stiernhielm (17th century), who was the first to write classical poetry in Swedish; Johan Henric Kellgren (18th century), the first to write fluent Swedish prose; Carl Michael Bellman (late 18th century), the first writer of burlesque ballads; and August Strindberg (late 19th century), a socio-realistic writer and playwright who won worldwide fame. In Sweden, the period starting in 1880 is known as realism because the writing had a strong focus on social realism. In the late 19th and early 20th century, other influential writers included Ola Hansson and Victoria Benedictsson. In the 1900s, one of the earliest novelists was Hjalmar Söderberg. The early 20th century continued to produce notable authors, such as Selma Lagerlöf (Nobel laureate 1909) and Pär Lagerkvist (Nobel laureate 1951). A well-known proletarian writer who gained fame after World War I was Vilhelm Moberg; between 1949 and 1959, he wrote the four-book series The Emigrants (Swedish: Utvandrarna), often considered one of the best literary works from Sweden. In the 1960s, Maj Sjöwall and Per Wahlöö collaborated to produce a series of internationally acclaimed detective novels. The most successful writer of detective novels is Henning Mankell , which has been translated into 37 languages. In the spy fiction genre, the most successful writer is Jan Guillou. In recent decades, a handful of Swedish writers have established themselves internationally, such as the detective novelist Henning Mankell and thriller writer Stieg Larsson. Also well-known outside of Sweden is the children’s book writer Astrid Lindgren, author of works such as Pippi Longstocking and Emil of Maple Hills.