The dlopen(3) man page. It explains the purpose and the use of the dlopen API.
Your favorite C++ reference about extern "C", inheritance, virtual functions, new and delete. I recommend [STR2000].
The Program Library HOWTO, which tells you most things you'll ever need about static, shared and dynamically loaded libraries and how to create them. Highly recommended.
The Linux GCC HOWTO to learn more about how to create libraries with GCC.