Try this link instead of that book: click here
It has all you will need to give an overview of almost every web language you might be interested in. It's also a lot simpler to get your head around than most books.
A very brief answer is that no, XML and XHTML are not the same thing at all. XHTML shares a lot of common ground with XML, but they are two separate things.
I'm cringing a bit at that explanation since there is so much involved to properly differentiate between XML and XHTML, but it will do for now to confirm that they aren't the same thing.