Scot recommends two very good books-- Robert Heine, Classical Christian Doctrine: Introducing the Essentials of the Ancient Faith, and Ben Quash and Michael Ward (editors), Heresies and How to Avoid Them. In addition to these, I would encourage you to read Alister McGrath's Heresy: A History of Defending the Truth. McGrath defines heresy as
A heresy is a doctrine that ultimately destroys, destabilizes, or distorts a mystery rather than preserving it. Sometimes a doctrine that was once thought to defend a mystery actually turns out to subvert it. A heresy is a failed attempt at orthodoxy, whose fault lies not in its willingness to explore possibilities or press conceptual boundaries, but in its unwillingness to accept that it has in fact failed.McGrath notes that in theology, doctrine "preserves the central mysteries at the heart of the Christian faith and life" (p. 30). That these central affirmations are mysteries is important. Doctrine is not, nor has it ever been, an attempt to explain and understand God exhaustively, as if that were possible. When St. Augustine says that if we can explain it, it isn't God, he is not suggesting that theological investigation and doctrinal explications are insignificant and unimportant. He is reminding us "that the human mind struggles and ultimately fails, to cope with the grandeur of God" (p. 29). But while our doctrine cannot disclose God exhaustively in God's grandeur, it can and must disclose God decisively in his character.