The Standard Template Library (STL).
I advice you the Josuttis The
C++ Standard Library as a reference/tutorial and the Meyers's Effective
STL to know how to apply it.
The
design and evolution of C++, Stroustrup. My preferred book. How the
C++ has evolved from the beginning to 1994. I do not think that you can understand
C++ without reading it.
Learn
assembler. Not absolutely necessary, but very helpful to understand any
programming language. Unfortunately books about assembler are out of date
or too much exhaustive. But one book, accessible freely, gives you a good
overview: PC Assembly
Language.
Learn
compiler writing. Not absolutely necessary, but very helpful to learn any
programming language. I found Introduction
to Compiler Construction accessible.
Elis&Stroustrup,
The annotated C++ Reference manual. Out of date, sure. And this is why
you have to read it! If you want to understand how the language has
evolved, and why there are so "interesting" features in C++.