Most of the documentation/tutorials you find will be perfectly relevant, but they may be Windows-centric based on .NET's history.

At a really basic level, you simply declare a "static extern" method with a "DllImport" attribute attached to it. For example, you could call "printf" from libc like so:

  [DllImport("libc")]
  public static extern void printf(string message);

  static void Main()
  {
      printf("Hello World");
  }