SharpDevelop was created as an example of how to create a high-powered GUI using C# for the old Wrox Press book, Dissecting a C# Application: Inside SharpDevelop. As I went to look up the book, I found that, since Wrox Press is no more (my book is technically published by Wiley, who bought some of the Wrox assets, including some of their titles), the book has been made available for free through Apress, who reserved the copyright for all the books Wiley didn’t buy the rights to.
You can download the entire book and SharpDevelop’s source code. I’ve just started reading it, and this is great. It basically shows you how to create an IDE that looks and acts like Visual Studio! Kudos to the authors, and thanks to Apress for making it free.