I am looking for a decent portable C++ (meaning C++ API, not C) GUI
framework that works both on Linux and Windows, so as to study and learn
it. That is, there should be at least one good decent book about it.

One option is QT. QT licensing however, is to pay for the QT framework,
for running non GPL applications on Windows.

Does anyone know any decent (like QT) portable C++ GUI framework for
Linux and Windows with good documentation (books) available?