class Book {
    final String name
    File sourceFile

    Book(String name) {
        this.name = name
    }
}

class DocumentationPlugin implements Plugin<Project> {
    void apply(Project project) {
        // Create a container of Book instances
        def books = project.container(Book)
        books.all {
            sourceFile = project.file("src/docs/$name")
        }
        // Add the container as an extension object
        project.extensions.books = books
    }
}

apply plugin: DocumentationPlugin

// Configure the container
books {
    quickStart {
        sourceFile = file('src/docs/quick-start')
    }
    userGuide {

    }
    developerGuide {

    }
}

task books {
    doLast {
        books.each { book ->
            println "$book.name -> $book.sourceFile"
        }
    }
}