Well, I'd advise taking a look at my GUI's source code (for Blitz3D) and Syntax Error's (known as JimB now I think) Sprite Control library.
GUI
The source for it is 99% uncommented, but most of it is fairly easy to grasp since it's all done with a simple concept: interpolate between two points to get the 3d on-screen point instead of camera picking a plane every time you need the point. Then, simply adding vertices to these 3d points and creating a quad- or panel/sprite/window/what have you.
Sprite Control
And this one.. well it's just really good.
GUI
The source for it is 99% uncommented, but most of it is fairly easy to grasp since it's all done with a simple concept: interpolate between two points to get the 3d on-screen point instead of camera picking a plane every time you need the point. Then, simply adding vertices to these 3d points and creating a quad- or panel/sprite/window/what have you.
Sprite Control
And this one.. well it's just really good.