I want to start developing GUIs for my application. My problem is, I use WIndows, OSX, and Linux and would like to use a portable GUI API so I can make one program and freely compile it onto the other platforms.

My question is, what is the best one to use? Which one has the best documentation (a book would be preferable so I would always have a refernce)?

I know of wxWidgets but could never get it to work correctly with the examples even when I followed the directions and qt requires a developer to buy a license if they wish to sell a piece of software made with qt. Are there any alternatives to these or should I re-try one?

Thanks