Black Books

"Where the Black Books actually came from... no one really knows. Some appear to have been written in the past, others might be from the future."

- Neloth Black Books are tomes of esoteric knowledge found in. There are seven different Black Books scattered through the ruins of Solstheim. These Black Books are Daedric artifacts of Hermaeus Mora.

Background
Black Books are esoteric tomes that are magical and other-worldly in nature. When read, they present the reader with exceptionally powerful knowledge that comes with incredible powers which mortals could never conceive. They are Daedric artifacts created by Hermaeus Mora to lure mortals into his service. Some appear to have been written in the past, while others appear to be from the future. They are much sought after as they contain hidden knowledge that grants the reader great power.

The Dragon Priest Miraak came into possession of one book that led him into the service of Hermaeus Mora. Eventually this resulted in him learning how to bend the will of Dragonkind. The Dwemer discovered another book in the First Era. Since the acquisition and usage of hidden knowledge was somewhat a specialty of the Dwemer, and they began to study it to reveal its secrets.

Reading
Opening a black book transports the reader into a region in the realm of Apocrypha. Once the region has been traversed, a book awaits on a pedestal. Reading the book grants a unique Power or ability.

However when opening a black book inside Apocypha, the corresponding region of Apocypha would not open. Instead the player would be transported back to Solstheim. This can be used to exit Apocypha at any time without using(reaching) the hidden knowledge book typically set at the end of each Apocypha scenario. This is especially important in the case of reading Waking Dreams without activating the final main quest At the Summit of Apocrypha because after reaching the word wall for Dragon Aspect, there is no visible exit in form of interactive item while the door behind player is locked.

Black Books can only be read in Solstheim. Attempting to read a black book in Skyrim will result in the message: "A vision of Solstheim swims before your eyes and then is gone." When the message is acknowledged, the inventory closes and the surroundings shake briefly.

Trivia

 * Some followers react to the Dragonborn using a black book, making such comments as "I can't believe  you do that on purpose", "Not again", "I wish you'd stop doing that", or "That can't be good for you..."
 * When the Dragonborn is standing near a Black Book, strange, eerie noises can be heard.
 * A Black Book can be treated as a portable storage container while on Solstheim. Upon re-entering the "Winds of Change" the player is close to a chest and the exit. It is helpful to dump the heavy loot there while exploring, then take it back when near a merchant/permanent storage.
 * The Sallow Regent is likely a reference to Robert Chambers' The King in Yellow.

Bugs

 * Upon exiting Apocrypha, the sound of paper whirling in the wind will persist. Saving and reloading the game should fix this.
 * Upon exiting Apocrypha from The Sallow Regent, the burning sound of the darkness will persist. Saving and reloading the game, or going back to Apocypha and returning should fix this.
 * If the Dragonborn falls into the water it results in death. When the game reloads the autosave, their health will continuously go down, even though in Solstheim. Loading a previous save is the only known fix.
 * When you use the perks book to exit out of The Sallow Regent, your health bar continues to go down. On two accounts this was fixed by going back into the book, conjuring a sword, start fighting the lurker while taking damage from the black 'fuzziness' and then falling/jumping down. A message appears on top left of the screen saying 'you cannot fight'. When it autoloads you, your healthbar is returned to normal.
 * If the Dragonborn is stomped on by a Lurker  they can be flung into the air similar to a Giant. When they respawn in Solstheim they can end up unresponsive, walking into the ground or laying on their side. The only known fix is to load a previous save.
 * Upon entering or exiting Apocrypha, any spells, shouts, or powers equipped will be immediately unequipped.
 * The Dragonborn may be unable to leave apocrypha by reading a Black Book. To return to Solstheim, the Dragonborn can "die" by going into the water.
 * Sometimes there are two copies of Untold Legends. Attempting to re-enter Apocrypha by reading Untold Legends may not happen, thus permanently locking the player out of that part of Apocrypha. The only known fix is to reload a previous save.
 * Entering and Exiting Apocrypha with Dragon Aspects active may cause the Dragonborn's facial hair to become invisible.
 * Sometimes reading a Black Book will cause the player to enter into a non-stopping load screen.  Current "fixes" are to simply skip reading them until Bethesda releases a patch.