Do bonuses from magic items stack? It seems that if I have two items that should lower my AC by one equipped, like +1 chain mail and a +1 ring of protection, it just lowers by one instead of by two. I want to make sure I'm not missing something obvious, or if something with my game broke. Also, how do the bracers work? Those also don't like stacking with anything else.

AC bonuses from magical armour does not stack with Rings and Cloaks of Protection, only with the Cloak of Displacement.
Bracers work with items of Protection, though, but doesn't stack with armour.

Magical bonuses on shields also stack with armor. As do helms, but you won't find those until PoD.