Put together by Randy Varnell, in Docmaker format, it's somewhat like the Pfhorte Handbook. Not quite as complete, but my guess is any mapmaker (or wannabe mapmaker) wouldn't go too wrong getting both of these.
Pict file containing M1 Alien textures with Readme.
Pict file containing M1 Wall 1 textures with Readme.
Pict file containing M1 Wall 2 textures with Readme.
Pict file containing M1 Wall 3 textures with Readme.
Pict file containing M1 Control Panel textures with Readme.
Pict file containing M1 Scenery Objects with Readme.