Before we start customizing the autonav block, we're going to have a quick look at the different options and the output. It's very helpful to be familiar with all the block options as well as knowing the HTML output the block generates before you start extending the block.
If you have followed the book chapter-by-chapter, you'll have the autonav block included in your theme, more precisely in header.php of your theme. Since we're going to play with navigation, we should undo this modification; changing the options in header.php would be a bit annoying otherwise. If you're done with this chapter, you might want to put the code back in place; it's mostly to make it easier to work with the custom templates we're going to build.