Qt 5 Cadaques is a free on-line ebook about Qt5.
The entire collection of chapters covering Qt5 programming, written by Juergen Bocklage-Ryannel and Johan Thelin, is available here. All book content is licensed under the Creative Commons Attribution Non Commercial Share Alike 4.0 license and examples are licensed under the BSD license.
It also provide offline ebook in ePub, PDF and QtHelp format.