A brand book is the document that sets out how to use a brand's visual identity: logo construction, colours, typography, clear space, and examples of correct and incorrect use.
A brand book is the brand's instruction manual – for employees, agencies and printers. Thanks to it, whoever prepares a banner, presentation or branded gift doesn't have to guess which shade or typeface to use.
Typical contents: logo construction and variants, minimum sizes and clear space, colour palettes (CMYK, Pantone, RGB, HEX), typography with hierarchy, combination rules, and examples of misuse.
Brand book vs design system: a brand book describes rules (you read it); a design system delivers ready digital components (you use it). Modern brands usually need both.