Hey Will,
This really depends: are you asing about refactoring the code-structure (classes, inheritance, is-a vs. has-a owenership etc) or actual "code" in the sense of stl vectors and refactoring actual "useful" code as I see it :)
For basics of C++ std libs etc, I generally refer to these:
http://www.learncpp.comhttp://www.cplusplus.com/reference/cmath/pow/ (or whatever function you're looking for)
In terms of design and such, I've read a lot of different books, but found it difficult to find really relevant things: most books focus on ATM machine class-structures etc (i know the knowledge applies across the board... but still)
There's also some Digital Audio & Java books I've read: in terms of general audio etc its useful: