code: function buildBook() {
    var fs = gitbook.createNodeFS(rootPath)
    var book = gitbook.Book.createForFS(fs)
    var Parse = gitbook.Parse
    var Output = gitbook.Output
    var Generator = Output.getGenerator('website')
  
    return Parse.parseBook(book).then(function(resultBook) {
      return Output.generate(Generator, resultBook, {
        root: outputPath
      })
    })
  }
docs: build book