Book of Medivh
Medivh's spellbook was kept in the Royal Library of New Stormwind following his death. It was retrieved by Alterac agents and taken to their kingdom. Teron Gorefiend with the help of Deathwing made a deal with Alterac king - Aiden Perenolde - and got the book in exchange for destroying Alliance outposts in Alterac. Eventually the book ended up with Ner'zhul, who also obtained the Jeweled Scepter of Sargeras, the Skull of Gul'dan, the Eye of Dalaran and used them to open many portals on Draenor. Ner'zhul's portals ripped Draenor apart. Obris held the book on the verge of death, and handed it to Khadgar. Khadgar used its knowledge to destroy the Dark Portal from Draenor's side, thus protecting Azeroth from the destruction of Draenor. But before the Portal was closed, Khadgar sent a Wildhammer gryphon rider with the Skull and the Book back to Azeroth, with orders to give them to the Kirin Tor.
The gryphon rider was successful in his mission as both the Book of Medivh and the Skull of Gul'dan were delivered into the custody of the Kirin Tor and were placed in a secure vault within Dalaran. The Book of Medivh was noted to have contained an extraordinary amount of power and knowledge. During the Third War, the Kirin Tor's possession of the Book of Medivh would lead the Scourge led by the deathknight Arthas Menethil and the lich Kel'Thuzad to launch a siege against Dalaran. After the siege was done the Kirin Tor's leader Archmage Antonidas would be dead and the Book of Medivh in Kel'Thuzad's possession. Drawing on all the power contained within the Book of Medivh allowed Kel'Thuzad to use the influx of power to crate a rift that linked Azeroth and the Twisting Nether. From this rift the Burning Legion led by Archimonde the Defiler were able to enter Azeroth.
After Archimonde and the Legion left for Kalimdor, the book remained in the hands of Kel'Thuzad.
This section concerns content related to Legion.
Kel'Thuzad apparently gave the book to his Cult of the Damned, and was stored by them inside Scholomance. On orders from Gul'dan, members of the Shadow Council led by Jergosh the Invoker retrieve it from Caer Darrow. Gul'dan intended on using the book, along with the Scepter of Sargeras and Eye of Dalaran, to open rifts through which the Burning Legion can invade Azeroth. Jergosh is killed and the book taken from him by a warlock adventurer.
In Karazhan the Shade of Nielas Aran, the involuntary father of Medivh, has , which could also be considered a "Book of Medivh". Mages from the Violet Eye - a sect of the Kirin Tor sent to spy on Medivh - were looking for the book and went mad when they read it. It was revealed that Kamsis gave the book to the Shade of Aran. When the player reads the book at the Master's Terrace they can watch into the past how Medivh killed the mighty blue dragon Arcanagos, now better known as Nightbane.