Currently there are no many books about Qt5, so main source is official documentation and web, but currently available books:
General Qt book which was updated to Qt 5.3, but in russian: Qt5.3. Professionalnoe programmirovanie na C++
No, I know people who don't use QML at all, but someone else use it more than C++. It really depends on your needs.
C++ is active and for example Digia offers additional not-free modules, like Qt Charts which can be used botj with QML and C++. Also Qt developers always support new C++ standards. For example we have additional features when C++11 or C++14 was released. Hence, it is definitely not a deprecated language.
QML is just a front-end language, it is very good to build beautiful interfaces without headache, but back-end is still C++ code.