The C++ codebase was split into multiple repositories in October 2015.

They are all gathered as git sub-modules under the webthree-umbrella repo.

There is a plan to re-consolidate the repositories following Homestead, which should make the structure of the codebase much easier to understand.

Significant effort was put into new Ethereum Documentation for Homestead in March 2016.

There is also an Ethereum C++ Wiki, which will probably be consolidated into the main documentation in the near future.