Cicero's final work is a masterpiece --
http://www.constitution.org/rom/de_officiis.htm
If you're used to reading books that spoon-read you rapidly without thinking, this might not do it for you. You need to read a paragraph slowly and analyze what lessons can be taken out of it, thinking about people you know that follow the precepts and those who don't, and getting takeaways from it.
But if you do that, I almost guarantee it'd give you new mental models and help identify some character flaws or operating errors you've got.
It's excellent, and a fast read. Knowledge of the history of end of the Roman Republic / start of the Empire helps give it context, but isn't necessary to get something out of the work.