Jan Oscar Sverre Lucien Henri Guillou (born 17 January 1944) is a Swedish author and journalist. Among his books are a series of spy fiction novels about a spy named Carl Hamilton, and a trilogy of historical fiction novels about a Knight Templar, Arn Magnusson. He is the owner of one of the largest publishing companies in Sweden, Piratförlaget, together with Liza Marklund and his common-law wife, publisher Ann-Marie Skarp.

Guillou's fame in Sweden was established during his time as an investigative journalist. In 1973, he and co-reporter Peter Bratt exposed a secret intelligence organization in Sweden, Informationsbyrån (IB). He is still active within journalism as a column writer for the Swedish evening tabloid Aftonbladet.

In October 2009, it was revealed that Guillou had been recruited by the KGB in 1967. The exposure of his activities came after the tabloid Expressen requested the release of documents from the Swedish Security Service (Säpo) and published information from the Säpo files along with information gained through interviews with former KGB Colonel Oleg Gordievsky about Guillou's case. The records showed that Guillou's involvement with the KGB continued for five years, until 1972.

From Wikipedia