Yes, it stacks as a "+" and is not multiplied. An example is if you have four items granting +1% exp then you will get a total of +4% exp. Keep in mind that items like this are useless after level 36 and should be substituted with something more effective in combat. The max level is level 40 but the bonuses received in skills and abilities or miniscule.
Exp bonuses apply to all sources (dispelling, lockpicking, quests, defeating enemies, etc,...). I have consumed exp potions to defeat enemies and simultaneously receive bonus exp from finishing a quest. By applying Reckoning mode and exp bonus potions you can gain about 30k exp multiple times in the game.