Most importantly, code something in c++! Being "book-smart" with nothing to show for it is pretty much useless. A simple Snake or Tetris clone in SDL is a super rewarding first project, or you could give Advent of Code a go as it lets you solve a lot of cool puzzles that fit in a couple of hours at worst.
As for books, I can recommend Effective Modern C++ by Scott Meyers. CppCast is a great podcast that can get you up to speed on the C++ ecosystem.
Why exactly? Current mainstream programming languages have just started to move away from the “fancy C” paradigm (only named functions, classes, imperative control flow constructs) to “prototype ML” paradigm (lambda functions, records, user-defined operators). These features make languages way more expressive (enabling things like SwiftUI), so understanding language nuances will become even more important as more advanced language features find their way into popular languages. Obviously you do not need to be a an expert on compilers, but most powerful languages need some time investment in order to be used at their fullest potential.