Different [1] is one of my favorite books on how to find a unique niche in crowded markets. It is written by a (brilliant) marketing professor but is useful for product strategy. Also see Blue Ocean Strategy [2] on this topic.
Essential had neither of these approaches.
[1] https://www.amazon.com/Different-Escaping-Competitive-Youngm... [2] https://www.amazon.com/Blue-Ocean-Strategy-Expanded-Uncontes...