These terms are used interchangeably in most cases. There might be slight differences in the contents of each document, or resource. The common purpose for all is to hold the brand elements and assets. Brand Guidelines are comprehensive in the sense that they include both visual and verbal guidance to implement a brand's presence out there. Brand Books can be even larger documents that detail specific use cases, and examples of implementation on many channels for both client-facing and internal communications. And Style Guides are typically more focused on creative assets such as graphic design, photography, videography, and voice and tone for copywriting. From here on style guides can go even more granular as Visual Style Guides, Literary Style Guides, UX Design Style Guide, UI Patterns Guide, and so on.