The book Outhink The Competition[1] really inspired me to start down this path. There's some good stuff in there, and that led to me finally picking up my copy of Porter's Competitive Strategy[2] and digging in. I take this seriously enough now, that I wrote a quick and dirty internal app just for managing competitive intelligence and keeping track of documents and links related to competitive analysis and strategy formulation.
Which, when you think about it, is a Good Thing. I mean, if you're talking to an investor and they say "how do you intend to compete with $FOO?" It's probably good to have a solid understanding of exactly how you intend to do that, instead of just saying "Oh, we'll out execute them." Especially when you choose to play in crowded markets like we do[3]...
[1]: http://www.amazon.com/Outthink-Competition-Generation-Strate...
[2]: http://www.amazon.com/Competitive-Strategy-Techniques-Indust...
I got a lot out of reading Outthink the Competition and am planning to read it again soon. I like his ideas about "coordinating the uncoordinated" and "take what's left behind when the battle moves elsewhere" (paraphrasing in both cases).
[1]: http://www.amazon.com/Outthink-Competition-Generation-Strate...
[2]: http://www.amazon.com/Hide-Dagger-Behind-Smile-Competitive/d...