Everyday Information Architecture
Quality primer on how to set up a system for evaluating and restructuring web sites. One of the better skinny Book Apart books I've picked up. I will say that at a startup I don't find this problem super pressing; it's rather for very mature sites—or sites with a plethora of content—that the exercises in this book become valuable. For those sites, you can think of this as a brief primer on outlining, akin to Chapters 5–7 of A Manual for Writers.
Martin, Lisa Maria. Everyday Information Architecture. New York, NY: A Book Apart, 2019. Printed Book.