Microsoft help bootstrap
Find the right Help Authoring Tool (HAT)
Best reference for HTML, XML, CSS, JavaScript
Best topic source editor?
  • There are several good free editors available (Google search). MS Expression Web is very good. Apparently Visual Studio 2012 XML/HTML editor works well. 

    Please don't use MS Word as a HTML Editor. It generates very dirty DHTML that you won't be able to maintain or easily control. 

    HTML Help (.chm) requires a very basic HTML editor (really NotePad will do the job if you know basic HTML). While the latest "Help Viewer" (VS 2010, 2012 help) requires something more advanced that can check XML syntax (closing tags and matching case etc).