The Return on Investment of Documentation and Support
http://www.cherryleaf.com/2008/03/could-technical-authors-help-you-get.html

Pratt, Ellis
Cherryleaf
2008

Abstract:

The benefits of user documentation (reduced support calls, increase in the perceived value of the product, happier customers, better customer retention, increase product usage etc) can be identified, but it can be hard to measure them and accurately quantify the Return on Investment.