Does Armor Stack?
In general, there are two notations for AC:
The most common variety sets your AC to a fixed value, and looks something like this:
AC = 11 + Dex Modifier (Padded armor)
Your AC equals 10 + your Dexterity modifier + your Wisdom modifier (Monk Unarmored Defense class feature)
Alternately, some items increase your existing AC, and look more like this:
Wielding a shield increases your Armor Class by 2 (shield)
... You gain a +1 bonus to AC (Fighter Defense fighting style)
When calculating your AC, you get ONE item of the first kind, and as many of the second kind as you can justify (except where contradicted by rules for specific items, e.g. you can only use one shield).
So if you have a Dragon sorcerer/barbarian, in full plate, with an extra set of padded armor under it, you have to pick which of the four forms of AC you use (Draconic Resilience, Unarmored Defense, Plate armor, or Padded armor). They do not stack.
But if you strap on a shield, that will stack because it grants +2 AC instead of setting your AC to a value. You can even strap on a cloak of resistance for another +1 AC.
See also:
What can I wear with Plate?
The rules state that plate armor consists of the following:
A suit of plate includes gauntlets, heavy leather boots, a visored helmet, and thick layers of padding underneath the armor.
There are no rules for substituting one piece of armor for another (e.g. a helm for a crown).
With that said, it is typically traditional to allow replacement of peripheral items (such as gloves, helms, boots, and bracers).
Yes
In D&D5 everything with a plus/minus stacks with every other thing with a plus/minus.
Things that set a value do not stack with other things that set a value but do stack with things that give a plus. For example, the Mage Armor spell sets your AC to 13 + DEX bonus; it does not stack with a draconic sorcerer's natural armor which also sets your AC to 13 + DEX bonus, however, either will stack with the Shield spell which adds +5 to your AC.
Best Answer
It stacks, depending on the specific items, up to AC -10
First off, AD&D 2e did not yet have the concept of a general rule about limiting bonus stacking. You have to look at each individual magic item to understand how it combines with others (which means, by default there is no rule against them stacking, if the item does not say so, as each item provides the bonus it says). The only limit is that the lowest attainable AC is -10.
In your example
That is, this would add up to +4 bonus (with book items), or +6 (with a newly introduced magical helm that improves armor class by +2).
Armor & Shield. The rules for magical armor (p. 181, DMG) say
So AC bonuses from armor and shield combine. Shield only is a type of armor, too (providing AC 9), so it counts as armor for effects that say they cannot combine with armor.
Helmets. In D&D, including 2e, helmets do not normally count as a separate armor item, they are part of armor (p. 76, PHB).
The available magical helms therefore do not change AC class, they provide other effects, like comprehending languages, changing alignment etc. If there was one which modified AC (there isn't), it would stack.
Bracers of Defense say
So they would not work while you also wear armor.
Ring of Protection says
So it does not work while you wear magical armor (but it would work on top of mundane armor).