I've slowly been building up a quick-reference guide of useful tips and notes for level design in Team Fortress 2 and Open Fortress. It includes notes on concepts like visibility and lighting cues, notes on entity quirks and useful console commands, explanations of engine things like tjuncs and edicts, and links to other useful resources.

This is an ongoing projecct - if there's anything missed or amiss, feel free to tell me.

Hope it helps!