Choose a new feature of the language -- creating random numbers (NOT using rand/srand) or dealing with std::strings, for example -- and write some test programs to push your understanding.
Reading books are one way to improve your understanding of C++, but not for the newer features of the language. I don't know of any well written book that does more than just a brief overview of C++14, and none touch C++17.
If you choose to spend time reading books don't buy just one. Or even one type.
https://isocpp.org/wiki/faq/how-to-learn-cpp#buy-several-books