mdBook is a program that creates "online book-like websites," from directories of Markdown files. It is particularly well-suited to creating online manuals and is used somewhat widely for that purpose in the Rust community. The mdBook user guide is an example of one such website. A number of plugins are available that extend mdBook with additional features, like rendering of LaTeX equation notation, checking links within a book for validity, and producing EPUB output. Detailed API documentation linked from the User Guide describes how users can write their own plugins in Rust. In the Format section of the User Guide, users will find details of the file structure that mdBook expects and information on how to customize the visual theme of the output. The Installation section of the mdBook site provides binaries for Windows, macOS, and Linux systems.