Most armor entries are unlocked by reading the relevant crafting books, some are tied to talking to/killing certain NPCs

Read "The Armorer's Encyclopedia" - ("The Craftsman's Manual" has been renamed)
- Hide
- Steel
- Iron
- Leather
- Fur
- Imperial (Also unlocked by talking to/killing Legate Rikke)
- Stormcloak(Also unlocked by talking to/killing Galmar Stone Fist)
- As of patch 1.2 - This book unlocks MOST other entries as well (Exceptions being a few Quest related armors no one should really have in a book - Ancient Shrouded, Ancient Falmer, Linwe's Armor, Blackguard Armor)


**If you haven't found the "The Armorer's Encyclopedia" yet, armor entries can also be unlocked by the below methods**


Read "About Elven Craftsmanship"
- Elven

Read "An Introduction to Malachite"
- Glass

Read "An Introduction to Scale Armors"
- Scale
- Chainmail

Read "Daedric Craftsmanship"
- Daedric

Read "Harder than Steel - How to work with Ebony"
- Ebony

Read "The Compendium of Draconic Armor"
- Dragonscale
- Dragonplate

Read "The Art of Dwarven Blacksmithing"
- Dwarven

Read "The Almanac of Orcish Blacksmithing"
- Orcish

Read "Art of the Nordic Blacksmiths"
- Quicksilver (Nordic)

Read either "Ice and Chitin" or "Blacksmithing of Morrowind"
- Chitin

Read "Blacksmithing of Morrowind"
- Bonemold


Read "The Rise and Fall of the Blades"
- Akaviri (Blades)

Read "Crafting with Netch Leather"
- Netch Leather

Speak to Baldor Iron-Shaper during the quest "A New Source of Stalhrim"
- Stalhrim

Kill Falmer
- Falmer

Talk to Isran at Fort Dawnguard
- Dawnguard

Talk to a Vigilant of Stendarr or Vigilant Tyranus in Markarth
- Vigilant

Kill Vyrthur during the Dawnguard main Questline
- Ancient Falmer

Kill/Talk to a Vampire
- Vampire

Read "Children of the Sky"
- Ancient Nordic Armor

Loot the Assassin of Old (Dark Brotherhood Quest from Gabriella after completing "Breaching Security")
- Ancient Shrouded Armor

Read Glover's Letter (In the basement of Glover Mallory's home)
- BlackGuard Armor

Talk to Baldor Iron-Shaper,Fanari Strong-Voice or  Storn Crag-Strider
- Skaal

Talk to/Kill a Golden Saint
- Golden Saint Armor

Talk to/kill a Dark Seducer
- Dark Seducer Armor

Read Sees-the-Moon's Journal (During saints & Seducers questline)
- Madness Armor

Read Amber Smithing Manual (During saints & Seducers questline)
- Amber Armor

Talk to any major Hold Guard
- Guard Armor

Talk to/Kill a Forsworn
- Forsworn Armor

Speak to Tonilia
- Thieves Guild Armor

Read "Nightingales: Fact or Fiction" or Speak to Nocturnal during the TG main quest
- Nightingale Armor

Talk to/Kill General Tullius, Commander Maro or a Penitus Oculatus agent
- Penitus Oculatus Armor

Talk to /Kill Astrid
- Shrouded Armor

Talk to/kill Linwe
- Linwe's Armor

Speak to Kodlak Whitemane
- Wolf Armor

Read "The Armorer's Encyclopedia"
- Redguard Elite Armor
- Silver Armor