A product manager is responsible for ensuring that a product or group of products meets the needs of customers over time. This role involves continuously monitoring and modifying elements of the marketing mix (product, price, place, promotion) to adapt to market changes, customer feedback, and competitive pressures. The product manager's duties include product strategy, roadmap development, feature prioritization, and cross-functional coordination to ensure the product's success in the market. Reference: "Product Management in Practice" by Matt LeMay provides an in-depth look at the responsibilities and functions of a product manager.