During the main quest of the Dragonborn DLC, you will encounter a couple of Black Books. When you read them, you will enter the realm of Hermaeus Mora, the Daedric Prince of Knowledge. By making your way through several chapters of the book, you'll come to the end and be granted additional power. These can be extra power to your shouts or the ability to reassign perk points.
There are seven of these books in total and two of them are collected during the main quest. most of the others are part of side quests on the island.
- Dragonborn Flame — When your Fire Breath Shout kills an enemy, a fire wyrm emerges from the corpse to fight for you for 60 seconds.
- Dragonborn Force — Your Unrelenting Force Shout does more damage and may disintegrate enemies.
- Dragonborn Frost — Your Frost Breath Shout encases foes in ice.
- NOTE: This book is claimed during the quest "The Path of Knowledge" but is part of the main quest line.
Black Book: Filament and Filigree
- Secret of Strength — Power attacks cost no stamina for 30 seconds.
- Secret of Arcana— Spells cost no magicka for 30 seconds.
- Secret of Protection— You take half damage for 30 seconds.
Black Book: The Hidden Twilight
- Mora's Agony — Summons a field of writhing tentacles that poison foes.
- Mora's Grasp — Freezes the target between Oblivion and Tamriel for 30 seconds, making them immune to all damage.
- Mora's Boon — Fully restores your Health, Magicka and Stamina.
Black Book: The Sallow Regent
- Seeker of Might — Combat skills are all 10% more effective.
- Seeker of Sorcery — All spells cost 10% less magicka.
- Seeker of Shadows — Stealth skills are all 10% more effective.
Black Book: The Winds of Change
- Companion's Insight — Your attacks, shouts and destruction spells do no damage to your followers when in combat.
- Lover's Insight — Do 10% more damage and get 10% better prices from people of the opposite sex.
- Scholar's Insight — Reading Skill Books gives you an extra Skill Point.
Black Book: Untold Legends
- Bardic Knowledge — Summons a spectral drum that plays for you, increasing Stamina Regeneration for you and nearby allies.
- Black Market — Summons a Dremora merchant to trade with you.
- Secret Servant — Summons a Dremora butler to carry your excess items.
- This book is claimed during the main quest line of the DLC and after you destroy Miraak at the Summit, you'll be able to spend a Dragon Soul to reclaim the perk points assigned to a single skill. Provided you have enough Souls, you could do this for all of your skills.