A Google search on 'melvyl api' generated a hit on an article that discusses the mechanics of Melvyl:

"For our initial implementation of the Google Book Search API (application programming interface), CDL integrated the API features into the Melvyl Catalog at the brief record display level."

This might give you a starting point for the API. For the actual MARC code, there is MARC-Record, which also may be a good starting point (the last updated field says '2007').