A specific document or a set of documents may be called a project. In Author-it, a project may be either a book or a whole library of related documents.
Many authoring tools are based around the concept of a project, in which all the components of a single document, perhaps in a couple of different output formats, are gathered together. Author-it's book object contains all of the output formats for a single document, and the Author-it Library is a collection of related books and shared components.