Seneca, Letters from a stoic
A man's worth is no greater than his ambitions.
If you liked Seneca, read "The Emporer's Handbook" by Marcus Aurelius. ([ame="http://www.amazon.com/Emperors-Handbook-New-Translation-Meditations/dp/0743233832/ref=sr_1_1?ie=UTF8&s=books&qid=1250880960&sr=8-1"]Amazon[/ame])
If you prefer headier erudition in your stoic books, read the one that started it all... Epictetus' "The Enchiridion".