Reading the reviews at Amazon and ACCU suggests that John Lakos' book, Large-Scale C++ Software Design may be the Rosetta Stone for modularization.
At the same time, the book seems to be really rare: not many have ever read it, and no pirate electronic copies are floating around.
So, what do you think?
I've read it, and consider it a very useful book on some practical issues with large C++ projects. If you have already read a lot about C++, and know a bit about physical design and its implications, you may not find that much which is terribly "new" in this book.
On the other hand, if your build takes 4 hours, and you don't know how to whittle it down, get a copy, read it, and take it all in.
You'll start writing physically better code quite quickly.
[Edit] If you want to start somewhere, and can't immediately get a hold of the book, I found the Games From Within series on physical structure useful even after reading Large Scale C++ design.