Doom Construction Kit
From DoomWiki.org
Doom Construction Kit (abbreviated as DCK) is a sector-based DOS level editor created by Ben Morris. It is similar to DEU in many respects; for example, DCK is mode-based.
Versions of DCK before v2.2f were free to download; versions after this must be bought to be used. However, older versions can still be downloaded and have the same features as later versions.
Book[edit]
A book, also titled The Doom Construction Kit: Mastering and Modifying Doom (ISBN 9781571690036) by Joe Grant Bell was published by Waite Group Press. The book explained how to make levels and other modifications for Doom. It was sold with a packaged CD-ROM containing DCK along with other editing tools including Dehacked, DMGRAPH and DMAUD.