
Editing Documents:
The Unofficial WAD Designers' Handbook
The Unofficial Doom Specs (62KB) by Matthew S Fell
The Doom Reference text (30KB) by Jim Flynn
Sector Tags, FAQ
Differences between Doom and Doom2:
Textures, Flats, Items, Sprites, and special features of each.
ZDoom Editing Tutorial (4MB) by Rick Clark and others.
The complete online tutorial guide (listed below) available as an offline version.