πŸ“š References + Updates (Categorized)

I. Official Documentation & Core Engine Info

LinkSubjectNotes
https://docs.unrealengine.com/udk/Two/SiteMap.htmlUE2 Site MapGood rounded reference for UE2 engines; some inaccuracies but still worth bookmarking.
http://wiki.tripwireinteractive.com/index.php/Console_Commands_(Killing_Floor)Console CommandsCommands for debugging and testing in-game.
https://wiki.tripwireinteractive.com/index.php?title=Killing_Floor_(modding)TWI Modding WikiTWI wiki with mostly unbroken links; good for mapping & modding, including ways to edit stuff without touching code.
https://docs.unrealengine.com/udk/Two/CreatingTerrain.htmlTerrain CreationCrucial link used in the Terrain section for basic setup and creation.
https://docs.unrealengine.com/udk/Two/EditingTerrainMaps.htmlTerrain MouldingCrucial link used in the Terrain section for sculpting and brush tools.
https://docs.unrealengine.com/udk/Two/EditingTerrainLayers.htmlTerrain LayersCrucial link used in the Terrain section for texture blending.

II. Mapping Guides & Advanced Techniques

LinkSubjectNotes
https://www.moddb.com/games/unreal-tournament-2004/tutorials/unreal-mapping-quick-start-guide-ut2004UT2004 BasicsBasics not covered in this guide (BSP fundamentals, etc.).
https://www.kf-wiki.com/wiki/Help:Mapping_Crystal_LakeOfficial Map Case StudyDetailed case study on designing a map using an official one as the base.
https://forums.tripwireinteractive.com/index.php?threads/kfs-containment-story-map-tutorial.80381/Objective Mode (KFO) GuideGuide specific to setting up Objective mode.
https://forums.tripwireinteractive.com/index.php?threads/how-to-optimize-your-map.43175/OptimisationOptimising your Killing Floor Map + other references in the thread.
https://wiki.beyondunreal.com/Legacy:XWeatherEffectAdvanced WeatherReference for complex weather effects (e.g., snow, rain).
https://forums.tripwireinteractive.com/index.php?threads/level-design-assets-and-tutorials-reference-thread.53850/Assets & ResourcesGreat post with tutorial links & free providers for textures, sounds, and models. Includes SFX resources.

III. Specific Actor & Component Guides

LinkSubjectNotes
https://lodev.org/unrealed/movers/movers.htmlMoversDetailed info on movers; noted that "making a lift" may not apply to this editor.
https://docs.unrealengine.com/udk/Two/MoversTutorial.htmlMovers TutorialCrucial link used in the Movers section for detailed settings.
https://docs.unrealengine.com/udk/Three/UsingWaypoints.html#Using%20WaypointsPathingTutorial and info for navigation points and pathing.
https://forums.tripwireinteractive.com/index.php?threads/setting-up-the-lobby-camera.110362/#post-2019092Lobby CameraCrucial link used in the Tips section for custom camera setup.
http://www.hourences.com/tutorials-semi-solids/Semi-SolidsCrucial link used in the Optimisation section for an alternative to additive BSP.
https://forums.tripwireinteractive.com/index.php?threads/how-do-you-make-slippery-floor-in-kf.56290/Slippery FloorsCrucial link used in the Tips section for material properties.

IV. Video Tutorials & Community Resources

LinkSubjectNotes
https://youtube.com/playlist?list=PLCCE44FF88A6409C1KF SDK Vids (Lethal Vortex)Probably the only guy who made more than 2 SDK vids just for KF1.
https://youtu.be/X6mi5Yh6vmkUE2 Tutorial Vids (General)This guy made $\sim$100 UE2 tutorial vids, and 95% still hold up for this editor version.
https://www.youtube.com/watch?v=XLspWWx3Qos&feature=youtu.beKF Speedmap TimelapseVideo of a KF Speedmapper in a timelapse.
https://www.insultplayers.ru/killingfloor/beyondunrealwiki/(start).htmlMonster ReferenceRehosted Monster Reference for UT2004/KF editors.
https://beyondunrealwiki.github.io/BeyondUnreal WikiAlternative rehost for the same material.
https://github.com/InsultingProsCommunity Tools & AssetsContains links to KF story map archive, other stuffs, and redirect tool.
https://www.rosswilding.com/design-resourcesLevel Design ResearchFor further research (mostly Level Design theory).