A used IBM Thinkpad, model 760XL laptop (P166, 80 meg RAM) running Red Hat 7.1. Sure, it's slow and has a funky keyboard, but it beats the heck out of a No. 2 pencil and a Big Chief tablet.
Bram Moolenaar's powerful SGML-aware vim text editor.
OpenJade, a DSSSL rendering engine for converting SGML documents into other formats.
DocBook, The Definitive Guide, by Norman Walsh and Leonard Muellner (O'Reilly, ISBN 1-56592-580-7). This is the standard reference for anyone attempting to write a document in Docbook SGML format.