I may be wrong but I don't think there is a theory of melody in as there is go harmony or counterpoint. A good melody can't be constructed from rules. That's what makes them magical.
As I write this, I think when a melody sounds good it's likely related to the implied harmony in the notes being used, and obviously the expectations the listener gets and how they're handled. But I don't think there is a system of constructing good melodies in Western classical music theory.
I'd say that once you understand practical harmony, counterpoint, diminutions, common schemata, some basic elements of form, you've pretty much understood what classical music theory has to say about melody too. There's definitely an element of playing with expectations in a fully "creative" and rule-free way, but knowing the theory underneath is how you understand what the expectations are.