Oxygen XML Editor Professional is an excellent tool that helps make single sourcing content a breeze. One place to edit all the content and output to PDF, e-Pub, WebHelp. It is fairly easy to customize the output, especially now with Chemistry, which goes a long way to keeping print and online content styles close to each other. I really love the fact that document generation time is reduced when producing multiple outputs simultaneously of the same content.

The documentation that goes with this tool is clear and full of excellent examples. If ever I get stuck, I know I will be fine if I read their guides