The Life Safety Code Handbook (7th ed.), published by the National Fire Protection Association. Over a thousand pages of pure fire-code goodness, together with exegetical commentary.
Have you ever wondered how prisons reconcile the need to keep the prisoners locked in with the need to get them out of the building quickly in case of fire? Or why some apartment buildings have metal gateways at the top of the stairs leading down to the basement? Or, most importantly of all, precisely how they come up with "maximum permitted occupancy" numbers?
It's all in the Handbook. And after just a few minutes of opening it to random pages, I know that I'm going to be looking at buildings with a whole new eye from now on. I'm realizing that all sorts of architectural decisions are determined by fire code rules. Those annoying doors segmenting perfectly nice hallways in my building at work, for example: they make sense now.