Sorting an XElement

I have a XElement that maps like follows:

<book>
    <author>sadfasdf</author>
    <title>asdfasdf</title>
    <year>1999</year>
</book>
<book>
    <author>asdfasdf</author>
    <title>asdfasdf</title>
    <year>1888</year>
</book>
<book>
    <author>asdfsdf</author>
    <title>asdfasdf</title>
    <year>1777</year>
</book>

How can I sort the books by author or title or year? Thanks