I'd also recommend OP2Utility considering what you want to learn about. Both projects contain details about things like the map file structure. As Brett said, OP2Utility took a lot of the guts from OP2Editor and reworked it into modern C++. The OP2Utility project is actively maintained, regularly compiled on multiple platforms, and should just work.
Back when OP2Editor was still being developed, we were using MSVC 6.0 (released in 1996). I don't know how well it would compile with a modern compiler. Meanwhile, the frontend of the op2mapper used Visual Basic 6.0. Microsoft never released a free version of Visual Basic 6.0. Couple that with the break in source code compatibility between VB6 and VB.net (effectively VB7) means we don't really have an easily accessible compiler to do development work on the op2mapper frontend.
With that said, I do see an OP2Editor.h in the repository. It's just kind of hidden among all the other similarly named files, such as the various project, solution, and workspace files, for multiple versions of Visual Studio, and the IDL (Interface Definition Language) file. As for how Visual Studio displays the projects, it tends to sort files into source files (.cpp), and header files (.h), so they may appear in different virtual folders within Visual Studio.
The COM/IDL/language interop stuff used to interface the OP2Editor backend DLL (written in C++) with the op2mapper frontend EXE (written in Visual Basic) does use a few intermediate topics, such as interfaces. The COM documentation on interfaces was actually a wonderful read. Though the rest of the COM documentation, on things like COM registration, factories, reference counting, and marshalling for DCOM (distributed COM) was much less pleasant (and much less relevant to the project). Anyway, hopefully the use of COM and IDL is not too hard to read, even if the details of how it all works is not too clear.
You're welcome to poke around with OP2Editor if you want. You might have an easier time with OP2Utility though.