Backlog size is inversely proportional to how often you talk to customers

Gayle McDowell Cracking the PM Career: The Skills, Frameworks, and Practices To Become a Great Product Manager (Cracking the Interview & Career)

Rob Fitzpatrick The Mom Test: How to talk to customers & learn if your business is a good idea when everyone is lying to you

Lenny Rachitsky 14 Habits of Highly Effective Product Managers