the C++ standard library (specifically the STL, N/A http://www.sgi.com/tech/stl)
recommended reading (C++)
anything "*Effective*" written by Scott Meyer
(invaluably helpful in everyday programming)
"Beyond the Standard Library. An Introduction to Boost" by Björn Karlsson
(shows you how to reuse boost correctly)
"Modern C++ Design" by Andrei Alexandrescu
(opens for you the gate to the fabulous world of templates. Gamma et al.'s design patterns are creatively revisited.)
"C++ Templates" by Nicolai M. Josuttis and
David Vandevoorde.
(You will find the building blocks of boost. Fasten your seatbelts, this is going to be a bumpy ride.)