Short version: A brand book is a practical guide that explains how a brand should be used and expressed.
It usually documents rules and examples for the logo, colors, typography, imagery, layout, tone of voice, messaging, icon style, spacing, and common applications. The purpose is not to make every asset identical, but to make the brand recognizable and easier to produce consistently. A good brand book helps designers, developers, marketers, sales teams, and partners make decisions without starting from zero every time.
The best brand books are usable, not just beautiful. They show what to do, what to avoid, and why the rules matter. For a small business, the brand book can be light and focused. For a larger company, it may become a full design and communication system with templates, component rules, campaign examples, and governance.
Related terms: brand identity, visual identity, and tone of voice.