I've read some other "modern" books about C like "21st century C" but they are not for people that do not know C and they are not so good for people that know it.
Can you provide any arguments to support what you wrote? What alternative you would propose for learning C?
https://www.amazon.com/Programming-Language-Brian-W-Kernigha...
Read what majority of opinions say. It's really easy to criticize without giving better alternative.
Er, well I was also using it as a mousepad for about three years...
If you are not being hyperbolic then please backup your 90% claim.
Meaning that many C++ applications, even nowadays, are actually C compiled with C++ compiler (not even classes are used).
So the transition between skill levels is quite gradual.
For me it is hard to tell, because I know the language since the C++ARM days, which means it had a feature size similar to Object Pascal and actually smaller than Ada.
Back then compiler writers even though it was easier to implement Ada compilers than C++ ones.
So slowly we got used to the new features, being discussed in books or programming magazines.