I recommend his seminal work Social Systems, though I can’t say how good the English translation is. I have read the German original, and it is difficult to read because its approach to its quite abstract subject is to explain and explore from many angles and refer to the body of prior work and their terminology without the benefit of math (though math wouldn’t help much I guess at this stage of the field).
A quick summary of Luhmann’s systems theory can be found on his Wikipedia page: https://en.wikipedia.org/wiki/Niklas_Luhmann#Systems_theory
I recommend his seminal work Social Systems, though I can’t say how good the English translation is. I have read the German original, and it is difficult to read because its approach to its quite abstract subject is to explain and explore from many angles and refer to the body of prior work and their terminology without the benefit of math (though math wouldn’t help much I guess at this stage of the field).