C++ is just so big. C++ has changed so much from 03 through 11, 14, 17, and now/soon 20 and 23. So much can come from external class libraries such as Boost. And the exact features you can use depend on your compiler, its vintage, and any legacy code you've got in your application.
Now, references, cppreference.com is excellent as a website.
I do see that Stroustroup himself has a 2018 book titled "A Tour of C++". I can't vouch for it, but it may be worth checking into.
IMO since C is the ancestor of C++ as well as ObjectiveC/Swift, C#, JavaScript, Java by learning pure C, you gain the foundations from which to learn not just C++ but those other languages.
Other languages can be transpiled to C (f2c from 1990 for Fortran, p2c from 1989 for Pascal) so cfront's C++->C conversion isn't really connected to the idea of superset-ness.
I've heard the argument that learning Latin first makes it easier to learn the Romance languages which derive from it. I don't believe it, in part because I found what helps me most to learn a language is actually having a need to use it (like visiting Germany for the first time after a year of German classes at school.) Start by learning the language you're going to use. Don't start with Algol simply because it's the historic foundation to C, Pascal, PL/1, etc.
Someone starting with K&R as a way to learn C++ will need to discard some styles, like using malloc/free. (And someone starting with Latin as a way to learn Spanish means discarding most of Latin's case system.)