Canon law refers to the law of the Catholic Church, which occupied a unique place in Western society prior to the Reformation, and influenced modern legal sytems, both civil and common law. (In England, canon law impacted the area of equity.) Canon law itself is largely derived from Roman law. The origin of "canon" is the Greek word kanon, meaning a ruler, i.e., a standard by which things can be measured. Up until the Code of Canon Law of 1917, canon law mainly resided in a series of compilations (largely of decretals) that developed over the centuries.
For more on the history of canon law, read this overview from the New Catholic Encyclopedia.
Corpus Juris (or Iuris) Canonici (Body of Canon Law)
Fourth Lateran Council of 1215: online version from Fordham University
Whether seeking print or online materials, here are some tips for using TALLONS, the online catalog:
Helpful call number ranges:
Helpful subject headings: