Necessary for SGML; useless for XML
This book is, regrettably, the one authoritative book on the SGML standard. Given how broad and confusing the SGML standard is, it's not surprising that this book on it is equally opaque -- this is, in my experience, the worst-written technical book I've ever seen that is not actually inaccurate. But if you're doing serious SGML development, you have no choice but to get this book and to spent forever trying to make sense of it.
But beware: if you're doing just XML, and if you think "well, since XML is a form of SGML, I might as well get the SGML standard", don't do it! XML is all you need to know, so just get an XML book-- I happen to like XML Pocket Reference, partly because it's less than one TWENTIETH the price of the SGML standard, and yet it's a hundred times more useful! (Having a copy of the XML spec, from the W3C site, can also be helpful.)
For the kind of money that you could be spending on The SGML Handbook, instead go buy a copy of the Codex Seraphinianus. It's equally indecipherable, but at least it's pretty.
Required SGML Reading IF
you are planning on really getting into the world of SGML. If you are a beginner, or just playing with SGML, this book isn't for you. The book does contain the entire ISO 8879 standard and is extensively cross referenced. After five years using it, I still find it easy to get lost in the references. If you want the final, definative word from the MAN who wrote the standard, this is it.
Turgid, obscure, confusing; but essential for advanced SGML.
This is a particularly badly written book on a particularly
badly designed and written standard, SGML.
However, SGML is so far the only reasonably universal and
standard way of marking up text, and this is the only
comprehensive treatment of it, including all the peculiar
little bits that you probably should never use. The book
includes the full text of the ISO standard as well as
cross-references and annotation.
The book, like the standard, uses terminology and notation
which are not standard in the rest of computer science. The
tutorial material is weak. The book design is ugly and hard
to read.
Yet SGML, bad as it is, is an important and useful standard,
and this is a comprehensive reference for it. Let us hope
that both the standard and the book will be improved radically
in the future.
The official ISO standard.
This is not the first book you should read about SGML, but if you are going to do any serious development in SGML, you need this on your bookshelf. It contains the entire text of the ISO standard, plus Goldfarb's annotations