My reading is hard to be divided in terms of number of papers,
since the Maya Developer Documentation, the book
"Complete Maya Programming",
and SDK examples were my primary source of information.
Thus, I will summarize in terms of chapters in the
documentation and the book that I read, instead of in terms of papers.

1. SDK Doc, Maya API introduction & Setting up your build environment
   - Describes the basic mechanics on how to create a plug-in DLL and
     load/unload it. Also describes some basic classes that are used.

2. SDK DOC, Command plug-ins
   - Describes how to create a command plug-in and how to support
     undo/redo according to the ways Maya does it internally.
   - Also describes ways to put your command in the UI.

3. SDK DOC, DAG Hierarchy
   - Describes how the scene hierarchy is organized inside of Maya.
   - Explains how DAGPaths should be used to uniquely identify objects.
   - Explains how Geometry objects don't contain transformation and
     how it uses its parents transformation

4. SDK DOC, Dependency Graph Plug-ins
   - Describes how to create objects that stay in the scene and
     interact with the scene. Doesn't have to be a DAG node.
   - Describes various types of DG Nodes that can be created.
   - Shows how to connect DG Nodes so they can interactively communicate.

5. Book, Chapter on DG Locator Nodes
   - Shows you how to create a Locator node that is visible in the viewport
     and can be transformed using the UI of Maya.
   - Describes how you can use OpenGL to draw it in anyway you wan into
     the viewport.
   - Locators aren't rendered. These are for objects like Camera, Light, etc.

6. SDK DOC, Manipulators
   Book, Chapter on Manipulators
   - This shows how you can add a graphic UI to Maya that makes manipulating
     attributes of an object more intuitive.
   - Translation arrows and rotations sheres are manipulators that are
     given by Maya. But you can create your own type and add it.
   - The drawing of your custom manipulator can be done using OpenGL.
     With some limitation, you can pretty much draw any type of graphic UI
     into Maya's viewport using manipulators.

7. SDK DOC, MEL Commands & API Classes
   - Essential dictionary for Maya plug-in development.