Where can I learn about editor programming?

I haven't found any thorough documentation on extending the Unity editor beyond what Joshan mentioned: Extending the Unity Editor. But that article and the scripting reference may be all you need after you understand a couple of core concepts. (That's how it turned out for me, at least.)

In one of your comments you seem to indicate you are trying to create a custom inspector, so that is what I will concentrate on in my answer.

Here are some core concepts that may help you get started:

Associating a Custom Editor with a Unity Object

When you create a custom inspector by extending the 'Editor' class, you need to associate it with another class that inherits from a Unity object. (That is key, that it extends a Unity object.) In the article, it may not be obvious, but the LookAtPoint extends MonoBehaviour. So it qualifies.

// The attribute is what tells the Unity editor when to use the custom inspector.
// In this case, use this custom inspector for LookAtPoint.
@CustomEditor (LookAtPoint)
class LookAtPointEditor 
    extends Editor  
{
   // Since this class extends 'Editor', it can be a custom inspector, assuming
   // you have placed it in the Editor folder.
}

The 'target' Field is the Object Being Edited

To gain access to the object that is being edited, use the 'target' field. If necessary, you can recast it to the class of the actual object. E.g. Recast to LookAtPoint. (This is important to know if you are using C#.)

Remember that you have access to everything within the target object. So if the object is a MonoBehaviour, you can use it to query the rest of the scene.

Intermixing GUI Layout and Code that Does Stuff

Once you get to the point where you can experiment, understanding the use of static methods to generate GUI elements should fall into place. Since the concepts behind GUI scripting is similar both in-game and in-editor, you can get a better understanding of it via the GUI Scripting Guide

An important item to remember is that you can intermix the code that does stuff with the GUI layout code.

if (GUILayout.Button("Build Mesh")
    && EditorUtility.DisplayDialog("Are you sure?"
        , "This will permanently change the mesh."
        , "Build mesh now."
        , "No, I am not ready."))
{
    // If the button is pressed and the OK option is selected in the dialog,
    // then call a horribly complex method that builds a mesh.
    BuildMesh(myMeshData);
}

Hope this helps.

There is a great tutorial up at http://active.tutsplus.com/tutorials/workflow/how-to-add-your-own-tools-to-unitys-editor/comment-page-1/#comment-50637

A good starting point for editor programming is: Extending the Editor.

If you're having trouble with static methods, one solution that I'm usually using is instantiating the objects as needed and then working with them. Depending on what kinds of objects you're dealing with, you might prefer to get references to the objects instead of creating new ones.