The Selfdocbook (XML edition) is held together by magic sed glue, which is activated by some rules in the Makefile in Appendix A, DocBook XML source. So, in order to build the Selfdocbook (XML edition) you will need make and sed (and also fold). Note that sed and fold won't be necessary for all SGML projects, but using a Makefile is usually a good idea.
To have a functioning XML environment, you will need to have various programs installed. On a Red Hat Linux 7.3 system, you need to install these packages:
See http://cyberelk.net/tim/docbook/ for more information.
The last package in the list there is xmlto, which is just a small shell script which invokes the other tools for you so that you don't have to remember which commands to run, how many times, and so on. It is similar to jw(1) from the docbook-utils package.
I use an emacs package called psgml for editing SGML or XML documents. The keybindings I find most useful are:
Tab indents the current element to a level appropriate to its nesting depth.
C-c C-t lists the valid tags for the current element.
C-c / closes the current element.