HTML is a subset of a more powerful language called SGML and the first question that spring to mind is why you need to use SGML when you are already happy publishing your Web pages with HTML? As this is probably the biggest barrier to buying the book a lot of it tells you why you would need to use SGML and the limitations of HTML. An example they give as to why you might need to use SGML is where you need greater control over the user's desktop or for automatic generation of tables of figures or contents.
The book starts with a small SGML application (displaying raw text) and gradually moving through more complicated examples.
The book comes with a CD, which contains numerous code examples and about 40 sample applications.
There is no arguing that SGML is a good language (standard - call it what you will). But it is one of those situations that you will learn it if you need to. For me, I am happy to publish my pages with HTML. If I need to create something more complicated then I might look at SGML but I'm not going to learn it 'just for fun'.
In summary, if you need to use SGML then this book will teach you how (and teach you well), but you need a requirement first.