Important Documents

Documents of Vatican II

Constitutions
Dei Verbum - DOGMATIC CONSTITUTION ON DIVINE REVELATION
Lumen Gentium - DOGMATIC CONSTITUTION ON THE CHURCH
Sacrosanctum Concilium - CONSTITUTION ON THE SACRED LITURGY
Gaudium et Spes - PASTORAL CONSTITUTION ON THE CHURCH IN THE MODERN WORLD

Declarations
Gravissimum Educationis - DECLARATION ON CHRISTIAN EDUCATION
Nostra Aetate - DECLARATION ON THE RELATION OF THE CHURCH TO NON-CHRISTIAN RELIGIONS
Dignitatis Humanae - DECLARATION ON RELIGIOUS FREEDOM

Decrees
Ad Gentes - ON THE MISSION ACTIVITY OF THE CHURCH
Presbyterorum Ordinis- DECREE ON THE MINISTRY AND LIFE OF PRIESTS
Apostolicam Actuositatem - DECREE ON THE APOSTOLATE OF THE LAITY
Optatam Totius - DECREE ON PRIESTLY TRAINING
Perfectae Caritatis - DECREE ON THE ADAPTATION AND RENEWAL OF RELIGIOUS LIFE
Christus Dominus - DECREE CONCERNING THE PASTORAL OFFICE OF BISHOPS IN THE CHURCH
Unitatis Redintegratio - DECREE ON ECUMENISM
Orientalium Ecclesiarum - DECREE ON THE CATHOLIC CHURCHES OF THE EASTERN RITE
Inter Mirifica - DECREE ON THE MEDIA OF SOCIAL COMMUNICATIONS

Other
Catechism of the Catholic Church - from the Vatican web site is a sure way to know the Catholic Faith. If you need to search the Catechism, here is a searchable version.  Also very helpful is the Compendium of the Catholic Church, which is in a question and answer format for easier understanding on the content of the Catechism..

Code of Canon Law

Papal Documents

Humanae Vitae

Liturgy

Pope John Paul II's document on the Mass - Dies Domini
General Instruction of the Roman Missal - the document that tells the procedures of the Mass.

My Articles

The Mass
Considerations about Communion in the Hand
Inclusive Language