Look at this link:
Sandcastle supported by Sandcastle Help File Builder - both found on Microsoft's CodePlex site.
A while back I was tasked with replacing our company's previous build-time documentation generation using NDoc (now deceased). Sandcastle was the proposed new method. After researching it, I found that Sandcastle is a mess of utilities produced by Microsoft to generate documentation. Trying to implement those utilities on their own is a nightmare.
Sandcastle Help File Builder is an incredibly useful layer that uses the Sandcastle utilities but makes the creation of a documentation project a million times easier. The developer behind it has been very responsive in the CodePlex forums when I've requested help.
It provides a GUI for configuring your project and produces a project file that is compatible with MSBuild. I set ours up to generate the SHFB project on the fly, run it with MSBuild, and output to our internal API web server. Works great.