You can always use inline style code for one-off formatting requirements.
But DON'T GET COMPLICATED! If your book is going to be any use, it must work on a range of devices. For instance, the Nexus 7 is becoming very popular. A commonly-chosen eBook reader for it, Aldiko, seems to ignore quite a lot of stylesheet instructions. It's often best to simply pour in your text, surround it by <p> tags, and let the device decide how it displays.