Since 1988 I have been teaching a course on SGML in the Department of Computing and Humanities at the University of Groningen in the Netherlands. The first year I taught from Martin Bryan's, SGML An Author's Guide to Standard Generalized Markup Language, Addison-Wesley, 1988. It was the book to be published on the subject and contains everything or nearly everything that is necessary to master the use of SGML. However, I found that it was difficult for students to use. One thing is that one must make heavy use of the index to find one's way in it.
When Eric van Herwijnen's first edition of Practical SGML, Kluwer Academic, appeared in 1990, I began to use it as my textbook. It was divided into three sections Getting Started with SGML, Advanced SGML, and SGML Implementations. The problem I found was that much information was not in this book which I had to supplement and the third section was not relevant to students who had to use SGML and write applications. The Electronic Book Technology version which appeared was a welcome addition in that while reading the book one could interactively use SGMLS and see what happens in an SGML process.
Charles Goldfarb's, The SGML Handbook, Oxford University Press, appeared the same year as Practical SGML. It was much more expensive, but indeed without doubt the SGML Bible. The publishers were aware of this fact and included two ribbon markers for users like in a Bible or missal to preserve relevant points. For experienced or good students even without much computing background this is both a starting and reference point. Yet for the average person not terribly computer literate, it can be overwhelming and inhibiting to use to actively use SGML. To do this one needs to read a more elementary book and try things out with SGMLS, the public domain parser by James Clark.
Now there is a new and much improved edition of Eric Herwijnen's, Practical SGML. It has been radically revised and is divided into 4 parts: Getting started, Writing a DTD, Customizing SGML, and Special Applications. The portion which I did not find useful with my students was 80 pages long; now this is reduced to 35. It is slightly shorter than the first edition, but much more thorough.
For anyone wanting to find out about SGML and use it, I can heartily recommend this second edition. It is clearly written and contains all that a beginner needs to know to use SGML. Of course, the Goldfarb book is and will remain the most important reference work for any serious SGML designer, but now there is a good introduction for someone just starting in SGML.
One of the most welcome additions in this book is a discussion in the final chapter on the standards, published and being prepared still, around SGML. These include SDIFF, HyTime, DSSSL, SPDL, and FONTS. This provides a perspective on how the larger picture which is being created by ISO SC18/WG8 for electronic text technology fits together. An internal document of the working group by Dr. James Mason does this also, but it has not been distributed to the outside world as far as I know.
As a member of the TEI team, there is only one thing which I miss in the literature. CONCUR was built into the original SGML standard, but implemented only by SEMA as far as I am aware. In dealing with electronically multiple hierarchies are a necessity and it would be very useful if we could make use of them. The editors of DSSSL have promised that it will provide a more powerful mechanism to do this. One hopes this will happen. At the moment all the examples used in DSSSL focus on printing SGML documents as far as I can see.
In this brief review of resources on SGML, I must also mention Yuri Rubinsky's, The SGML Primer, sold by SoftQuad. All the most essential aspects of SGML are treated in 35 pages, followed by a short bibliography. At the same time one learns how to write a DTD for Noah's flood and order and divide the bill at a restaurant while SGMLers are noting DTD fragments on the tablecloth.
Nearly lastly there is the Compleat SGML on CD-ROM from Exoterica. This is a hypertext version of the standard itself with many other things linked into it. This includes internal cross references, all proposals on emendations and revisions within WG8, and much more.
Lastly and most importantly of course for only 7 pounds more than Eric's book you can purchase the TEI Guidelines. Yet they are not competitive books; they supplement each other.