Sallust wrote his `history' of the `conspiracy' of Catilina between c.44 and c.35 BCE, 20 to 30 years after the events and he probably relied on Cicero's published speeches against Catilina. But whereas Cicero wanted to portray himself in the heroic role of the consul who foiled the conspiracy, Sallust was more interested reflecting on the past and present and applying the lessons from one to the other.
Here he is particularly interested in the concept of decadence, the anti-Roman values of Catilina's time which, in numerous Roman narratives, leads to the fall from past Roman austerity and virtue to present moral decline.
As well as being of intrinsic interest in itself, Sallust's prose is far more literary than Cicero's oral speeches. Ben Johnson used Sallust as the basis for his play Catiline (1611) and it might also have influenced Shakespeare's Roman plays: Coriolanus, Julius Caesar, and Antony & Cleopatra (though he also relied on Livy and Plutarch). Well worth reading and the Latin's not too difficult.