I´m currently working on an AutoCAD plugin using the .NET API, which Autodesk finally provides via NuGet. This is a big step forward because it makes it so much easier to get started with AutoCAD .NET development. Alas, the provided libraries come without XML documentation, so it´s always a game of hide and seek when you are looking for a specific functionality. In my eyes missing documentation is the single biggest problem of AutoCAD right now, which results in a steep learning curve for you and me.
For example, you can get an API reference that is published by Autodesk as part of the ObjectARX download. It is published in CHM format (EW!) but the bigger problem is, that it is far from complete. It is frustrating when you try to look up information about a method and you realize that only half of the members of the class at hand are covered, in the official docs!
So now to the good news! There are a couple of excellent sources of information:
Through the interface
Great blog by Kean Walmsley on AutoCAD. Typically two to three posts per week! I almost always find the information I need there.
A couple of blogs maintained by the Developer Technical Services team.
Autodesk community board for ACAD .NET
Subscribe to the RSS feed.
Apart from the virtually nonexistent official API documentation (please correct me, if I´m wrong) I actually enjoy working with AutoCAD. It is great to work on mathematical problems and it is far more rewarding to see the visual fruits of your hard labor compared to a typical forms-over-data enterprise application.
Autodesk also provides a download for the .NET API documentation that integrates with Visual Studio (Microsoft Help Viewer), but this contains the same amount of information as the CHM docs.