I was in a similar situation to yours when work required me to become a C++17 expert practically overnight. Bjarne (inventor of the language) has a "blue book"

I was in a similar situation to yours when work required me to become a C++17 expert practically overnight. Bjarne (inventor of the language) has a "blue book" that is very good and pretty comprehensive (if a bit dry and academic). I read it cover to cover in a couple days - not really retaining everything but learning what I didn't know. Then I read Scott Meyers's Effective Modern C++ to get up to date on the latest thinking and really nail down some of the finer points. YMMV but this worked well for me to get a good feel for C++ the language. Mastering the STL and/or Boost is a journeyman job, though, as is mastering (any) build system. Of course there is no substitute for firing up the editor and writing real code.