A whole section of Aquinas’ “Summa Theologiae” deals with moral theology, however it can be tough applying what he writes to today’s problems, and his reading is so dry for modern readers. Still, he’s the gold standard.
You can get an abbreviated edition called “The Summa of the Summa” by Peter Kreeft on Amazon.com if you don’t want to read the whole huge thing.
If it isn’t for you, let me know if you want something more modern. Send me a PM.