How to access grand child element?

I'm trying to learn XMLMapper, So I thought of using my W3 Schools XML example about books to try and map them my self.

So I'm trying to print title, author from an array of Books.

Books.XML

<?xml version="1.0" encoding="utf-8"?>
<bookstore>
    <book>
        <title>Everyday Italian</title>
        <author>Giada De Laurentiis</author>
        <year>2005</year>
        <price>30.00</price>
    </book>
    <book>
        <title>Harry Potter</title>
        <author>J K. Rowling</author>
        <year>2005</year>
        <price>29.99</price>
    </book>
    <book>
        <title>XQuery Kick Start</title>
        <author>James McGovern</author>
        <author>Per Bothner</author>
        <author>Kurt Cagle</author>
        <author>James Linn</author>
        <author>Vaidyanathan Nagarajan</author>
        <year>2003</year>
        <price>49.99</price>
    </book>
    <book>
        <title>Learning XML</title>
        <author>Erik T. Ray</author>
        <year>2003</year>
        <price>39.95</price>
    </book>
</bookstore>

BooksXMLMappable.swift file:

import Foundation
import XMLMapper

class BookStore: XMLMappable {
    required init?(map: XMLMap) {
        //Empty
    }

    var nodeName: String!

    var books: [Book]?

    func mapping(map: XMLMap) {
        books <- map["book"]

    }

}

class Book: XMLMappable {
    required init?(map: XMLMap) {
        //Empty
    }

    var nodeName: String!

    var title: String!
    var author: String?
    var year: Int?
    var price: Double?


    func mapping(map: XMLMap) {
        //category <- map.attributes["category"]
        title <- map["title"]
        author <- map["author"]
        year <- map["year"]
        price <- map["price"]
    }

}

Now I tried to run this in my ViewController's ViewDidLoad:

let storeObject = XMLMapper<BookStore>().map(XMLfile: "books.xml")
        print(storeObject?.books?.first?.author ?? "nil")

And I have successfully printed author for the first book, but I can't find a way to print author's of all the books without getting nil.

And if a book has multiple authors which one will be printed and how to print them all ?

Sorry if my question is too easy to solve, or a very basic one, but I'm trying to learn.

Thanks in advance.

used for in loop for print author like this

let storeObject = XMLMapper<BookStore>().map(XMLfile: "books.xml")
for book in storeObject.books {
      print(book.author)
}