First published in 1984, this book is the theoretical program for Luhmann's following works, his series about the subsystems of society: Science, Law (already translated into english), Art, Economy, Religion and Politics, each "as a Social System". The final synthesis is made in the monumental The Society of Society (published in 1997, not yet translated).
"Social Systems" presents a turning point in Luhmann's work: for the first time, he places people outside of societies' systems. They suffer the consequences of society, but they are not part of it. This has been said to be anti-humanist (or to represent an "autopoietic turn"); to me, it appears to be quite the contrary: People are more than their religions, legal systems, political convictions or scientific theories: they are people. Of course, the distinction between people and their creations leads to interesting problems, and Luhmann is the first to acknowledge that - and in this book, he thinks them through.
Another fundamental postulate of Luhmann's is that each of the subsystems of society works with a binary code, for example true/false for science, legal/illegal for law.
His work is influenced by an enormous amount of reading in each of the disciplines he discusses; however, the underlying epistemology is mostly inspired by radical constructivism as proposed by Maturana/Varela or Heinz von Foerster.
In the construction of his theory - which has continued to change in nuances after "Social Systems" - Luhmann tries to avoid tacit assumptions: he always says what he's doing, says which other possibilities exist and have been tried. His style invites other interpretations, i.e. the use of other distinctions which might enable us to see more interesting and useful things.
This, together with a fine sense of irony and a good dose of scepticism make Luhmann the most popular and widely read contemporary philosopher and sociologist in Germany.
The drawback: Luhmann's style sometimes sorrily lacks the polished elegance and simplicity of anglosaxon writers. Don't be discouraged: one of the 21st centuries greater intellectual adventures is waiting for you.