PySide is LGPL-licensed Python bindings for the Qt (framework for cross-platform UI and applications). PySide is currently maintaineed by the Qt company, under the name "Qt for Python".
PySide bindings allow both free open source and proprietary software development and ultimately aim to support all of the platforms as Qt itself.
Official site: https://wiki.qt.io/Qt_for_Python
Books
Create GUI Applications with Python & Qt5 (PySide Edition)
Martin Fitzpatrick - Published June 2020, CC-BY-NC-SA. 665 pages, 211 code examples
Basic principles to full applications, including advanced topics including Qt threads & processes, custom widgets, plotting, model-view MVC architecture, SQL databases and packaging complete apps on Windows, macOS and Linux.