Denzinger's famous Enchiridion Symbolorum et Definitionum is a handbook on the sources and development of the Catholic Faith. Like other typical German Handbuchs of the 19th century, this one is dense, hard-to-read, and very, very thorough. Dig deep, and you will discover what is doctrine, what is condemned, and what is open to interpretation, and gives you the who, what, when, and why, going all of the way back to the beginning.UPDATE: for whatever reason, Google likes to direct folks seeking the Enchridion to my web page here. An English version of the Enchiridion Symbolorum was once put online, but has since been taken down, but you can still find a copy of it on the Internet Archive here. It is also available in Latin, Spanish, and French.