If your "special" lines are not already marked in some way then you will have to manually go through the book to make changes. However, you can create a "clip" using the clip editor that will make this process much easier. All you need to do is highlight all the lines you want to "bundle" and then click on the clip to wrap the bundled lines in a container.
I use several but here are a couple of examples:
Div: <div class="">\1</div>
Block: <blockquote class="">\1</blockquote>
Once I use the clip then I just fill in the class I want to use between the quote marks. I definitely use MZMM's technique:
CSS (styled as you desire)
.poetry {margin:2em}
.poetry p {font-style:italic; text-indent:0}
HTML
<div class="poetry">
<p>line 1</p>
<p>line 2</p>
<p>line 3</p>
...
</div>