Good introduction in Finnish is Marko Pyhtilä's Kansainväliset Situationistit. In English there's a website and archive Situationist International online.
SI was a group established in 1957, as an artist avantgarde group; but from the beginning with a political intent, so much so that later the situationists rejected the artist definition and thought of themselves fundamentally as a political group. Situationists and their ideas were very prominent in the 1968 student and worker demonstrations - see the picture: Under the pavement, the beach. The ideas also echo in various movements and thinkers such as Deleuze and Guattari or Italian Operaismo.
Here is Marx's opening to the Capital:
"The wealth of those societies in which the capitalist mode of production prevails, presents itself as “an immense accumulation of commodities,” its unit being a single commodity. Our investigation must therefore begin with the analysis of a commodity."
And Debord's The Society of the Spectacle:
"In societies where modern conditions of production prevail, all of life presents itself as an immense accumulation of spectacles. Everything that was directly lived has moved away into a representation."
Visible the themes of capitalist mode of production and the alienation wrought by it; plus typical "detournement", re-cycling of existing texts, cultural tradition, without quotes: putting it to use. Other two basic concepts discussed were "dérive", wandering and moving without a goal, playful use of space (and time), creating situations, changing how we live; and "recuperation", the appropriation and re-appropriation of everything produced by people by the system, capitalist mode of production. Everything is turned into a spectacle, commodified, reified, alienated...