Here are some key references that can help you delve into product design as a product manager:

1. "Hooked: How to Build Habit-Forming Products" by Nir Eyal: This book offers insights into designing products that create user habits, which is essential for product managers to understand in order to drive user engagement and retention.
2. "The Design of Everyday Things" by Don Norman: Don Norman is a pioneer in the field of human-centered design. This book provides fundamental principles of design that are applicable to any product development process.
3. "Sprint: How to Solve Big Problems and Test New Ideas in Just Five Days" by Jake Knapp: This book introduces the concept of design sprints, a structured process for quickly prototyping and testing ideas. Product managers can learn how to integrate design sprints into their product development cycles for rapid iteration.
4. "Lean UX: Designing Great Products with Agile Teams" by Jeff Gothelf and Josh Seiden: Lean UX emphasizes collaboration and experimentation in the design process. Product managers can learn how to integrate Lean UX principles into their product management practices to deliver user-centric products efficiently.
5. "Design Thinking: Understanding How Designers Think and Work" by Nigel Cross: This book provides an in-depth exploration of design thinking, offering valuable insights for product managers seeking to foster innovation and solve complex problems through a human-centered approach.