These are links to other systems where you can find essential official Church documents as well as other helpful publications which don't fall into the "official" category. This is not an all-inclusive list.

All the Documents of the Second Vatican Council	Located at Christusrex.org
Constitution on the Sacred Liturgy	Sacrosanctum Concilium, 4 December, 1963 Located at Christusrex.or
Music in Catholic Worship	US Bishops, 1972
Musicam sacram (1967)	Instruction on Music in the Liturgy Located at EWTN
Pius XII: Musicae Sacre (1955) On Sacred Music	Located at EWTN
General Instruction of the Roman Missal	1975 Located at Christusrex.org
The Catechism of the Catholic Church	Located at Christusrex.org
Documents of the Catholic Church	Located at Spring Hill College
Gather Faithfully Together - A Guide for Sunday Mass	Cardinal Roger Mahony's excellent pastoral letters on celebrating Mass, the Eucharist and ministry. Located at the Archdiocese of Los Angeles
The Day On Which We Gather
As I Have Done for You - A pastoral guide to ministry
Reading the Liturgical Documents	Modern Liturgy, Sept. 1996
Liturgical and Biblical Texts for the Third Millennium	Bishop Donald W. Trautman on the revised Sacramentary and the revised NAB lectionary.
Liturgy Links	Links, links, and more links; many on music.
Notre Dame Center for Pastoral Liturgy
US Bishops’ Committee on the Liturgy
Liturgy.NET

Liturgy and Church Documents