There are so many encyclicals, apostolic letters, etc. One could spend years reading just a fraction of them, depending on reading and comprehension speed, of course, which varies by person.

Two I recommend, from the last 40 years:

Veritatis splendor, John Paul II, 1993

Argues that Christian freedom is fulfilled, not limited, by objective moral truth: some acts are intrinsically evil regardless of intention or circumstance, conscience must be formed by divine law rather than self-authorization, and the Church must faithfully teach this moral truth as the path to authentic human flourishing in Christ.

https://www.vatican.va/content/john-paul-ii/en/encyclicals/d...

Fides et Ratio, John Paul II, 1998

Argues that faith and reason are complementary paths to truth: reason needs faith to avoid skepticism, relativism, and reductionism, while faith needs reason to express, defend, and deepen its understanding of divine revelation and the human search for meaning.

https://www.vatican.va/content/john-paul-ii/en/encyclicals/d...