Canon Law can be the law of any religion, but the phrase most commonly refers to the law of the Roman Catholic Church.
The Canon Law Research Guide by Don Ford covers Catholic, Easter Rite, Orthodox, Anglican Lutheran and Mormon law.
Roman Catholic: Roman Catholic Canon law is codified in Codex Juris Canonici (i.e., the Code of Canon Law). The Vatican posts an English translation. Commentaries on the Code include the University of Navarre's Code of Canon Law Annotated and the Canon Law Society of Great Britain and Ireland's The Canon Law: Letter & Spirit (Liturgical Press).
The Canon Law Society of America publishes a translation of the Code of Canon Law as well as the Canon Law Digest, "a compilation of official documents relating to canon law."
The Columbia Law School Library has a substantial canon law collection and a first-rate document delivery service. The Biblioteca Pontificia Università della Santa Croce (the library at the Pontifical University of the Holy Cross) in Rome has an extensive collection of canon law collection and staff members with expertise in the area.
Letters and other writings by the Pope are posted in Papal Encyclicals Online.
The "Christian Canon Law" section of Religious Legal Systems: A Brief Guide to Research and Its Role in Comparative Law by Marylin Johnson Raisch discusses the basic sources of Catholic Canon law with links to online resources.
Eastern Canon Law: The canon law of the Eastern churches in communion with the Roman Catholic Church is codified in the Code of Canons of the Eastern Churches. An English translation is sold by the Canon Law Society of America.
Vatican City Law: The Vatican City State is governed by a mixture of Canon law, Italian law, and its own laws. For more about the VCS legal system see, Researching the Law of the Vatican City State by Stephen Young & Alison Shea and/or "Separating State from Church: A Research Guide to the Law of the Vatican City State," 99:3 Law Library Journal 589 (2007).