The C++ FAQ (Frequently Asked Questions) has a wealth
of information about object-oriented programming in general and
about C++
specifically.
If you
want the real low-down on what is and isn't legal, and can wade
through some fairly technical details, check the draft ANSI/ISO standard for
C++.
The VIDE documentation package is a useful collection of
manuals and help pages about both g++ and other programming
related subjects.
The C++ Standard Library (std/STL)
Tutorial & reference material from SiliconGraphics on the STL
(containers and iterators). Good stuff! You may want to bookmark
that page.
The DinkumWare
documentation
describes the portions of the std library that SiliconGraphics
does not (e.g., iostream, string).
My own reference sheets for containers and algorithms may be useful to keep at hand
when you actually begin programming.