The bible of competitive intelligence. Consult it for guidance on a wide range of problems. Herring isn't very enthusiastic about the Web as a medium for tracking the competition. He shared his counterintelligence with Fast Company. The best things in life aren't free.
Free information is usually secondary information.
And the problem with secondary information is that everyone else has access to it. It doesn't give you an advantage. You won't gain an advantage over other people unless you spend more time and money than they do. That means subscribing to syndicated services and databases - and, in some cases, paying people to work those databases. But ultimately, you want information that can't be found in any database.