This book isn’t particularly relevant with HTML5 now the standard. And, of course, XHTML came and went since this book was published.
However, it was a great book when it was published. It wasn’t a thick book, it was spiral bound and small. However, it was a nice tutorial and got me up and running creating HTML documents. HTML4 was a very relevant standard, even in the XHTML era because XHTML was based on HTML.
The book advocated good web design and best practices. Because it covered HTML 4 when it was a brand new standard, it captures a curious place in the annuls of web design and development. It talks about depreciated tags, the use of internal properties to style pages, as well as the nascent CSS standard.
It’s weird that the book is called a quick reference, because it’s not a reference book at all. It’s a tutorial. If all you want to learn is HTML5, I suggest a more modern HTML5 book like the HTML5 The Missing Manual. If you want to learn web development (a noble goal) I suggest a book like PHP & MySQL for Dynamic Websites by Larry Ullman.