A mammoth resource back in the day for creators looking to make their own levels for DOOM, DOOM II, and (to a much lesser extent) Heretic. Although the information on the various WAD authoring and editing tools is quite outdated (there are much, much better programs available for modern systems that also work under the enormous variety of source ports), if you're looking to mess around with the internal workings of DOOM then there's still plenty of information about level creation that is valid today. The book also spotlights a number of great WADs, points out some of the gimmicks in their level design, and explains how to use those same tricks and gimmicks in your own levels.
This is a massive book, nearly 950 total pages in size, including an 8-page color Gallery right in the middle. It also contains a number of pages which are completely blank; to help lower the file size, I elected not to include these blank pages in the scan.
This file also doesn't include the CD which came with the book, but those interested in checking it out can find it over at Archive.org.