You can create a form with 2 text box, one for title of book and one for author of book. I will assume that author's name is unique, the controller's code will be something like:
def saveBookForm(String title, String name) {
Author author = Author.findByName(name)
if (author == null) {
author = new Author(name:name).save()
}
new Book(title:title, author:author).save()
}