It doesn't matter which framework you use - it's a crime for any and all c++ programmers to not have the legendary trilogy (now, quadrilogy) from Scott Myers. Buy them. Period.

As for Qt, there are some fundamental aspects on how object (read, QObject's) lifetime is handled in Qt. Often using standard unique_ptr with Qt object paradigm creates issues. You need to understand the usage of Qt supplied smart pointers. Also a lot of classes/containers available in c++ standard are designed in Qt separately - which may appear quite redundant - but understanding why they are there and their relative advantages while using Qt framework is crucial during development processes.

I personally am pretty new to Qt and while I know I can learn the framework from its exhaustive documentation, but the lack of an offline/pdf/EPUB is a great downer. Plus, I am also a fan of good ol' paper textbook. Following are two suggestions: 1. An introduction to design pattern in c++ using Qt. 2. Advanced Qt programming (Summerfield).