•Why it earns a slot
Why it earns a slot: it is the last thing Cicero wrote before Antony's proscription killed him, and its central argument, that nothing genuinely useful can ever be genuinely dishonest, impressed Frederick the Great enough that he called it the best book on morals ever written.
Cicero's last work before Antony's men killed him is a father's field manual for right conduct, built on the claim that nothing genuinely expedient can ever be genuinely dishonest.
This distillation is written from the freely available original, which is always the better read when you have the time: Project Gutenberg.