"Competitive Strategy: Techniques for Analyzing Industries and Competitors" by Michael E. Porter is a highly influential book that provides several key lessons about strategy. Here are the main takeaways:

1. Understand the Five Forces: The book introduces the concept of the 'Five Forces' (industry rivalry, threat of new entrants, threat of substitutes, bargaining power of buyers, and bargaining power of suppliers) which shape the competitive dynamics within an industry. Understanding these forces can help a company to better position itself.
2. Choose a Clear Competitive Strategy: Porter presents three fundamental strategies to achieve a competitive advantage: cost leadership (becoming the lowest-cost producer), differentiation (offering unique and superior value), and focus (concentrating on a specific niche market). He suggests a company must choose one strategy to avoid getting "stuck in the middle" with no clear strategic direction.
3. Sustaining Competitive Advantage: It's not enough to gain a competitive advantage; a company must also sustain it. This can be done by continually improving and innovating, creating barriers to entry, and staying responsive to changes in the industry.
4. Analyze Your Industry Structure: Understanding the structure of your industry can help you to predict its future direction, identify opportunities and threats, and plan your strategy accordingly.
5. Consider the Value Chain: Porter emphasizes that companies should analyze their value chain — all the activities involved in producing, marketing, delivering, and supporting a product — to identify potential areas for cost savings or differentiation.
6. Don't Underestimate the Power of Suppliers and Buyers: Companies must pay attention to the bargaining power of their suppliers and buyers, as this can greatly impact their profitability.
7. Recognize the Threat of Substitutes: Substitute products or services can limit a company's profitability by placing a ceiling on prices. Monitoring potential substitutes is essential.
8. Be Aware of Barriers to Entry: The ease with which new competitors can enter your industry affects the level of competition and profitability. Companies can use barriers to entry to protect their position.
9. Monitor Industry Rivalry: The intensity of rivalry among existing competitors can affect a company's ability to raise prices and improve profitability.

In sum, "Competitive Strategy" teaches that the key to business success lies in understanding your industry, choosing a clear strategy, and working to create and sustain a competitive advantage.