Can be found randomly in loot containers, and is randomly sold by general goods and books vendors.
Vehk's Book of the Last Hour
"And in those waning days, I decided to go to Cyrodiil with my Water Face, so that I might tell all I met along the way the truth and the truth only. I was then the Thief of the world, and my charges were three (and that being a very significant number to me), one of which was the Tower.
"And it was of the Tower that my emperor wanted to hear. He was dying and I loved him yet. He, too, was a Master and so I knew that he realized just how big a realm that the Tower encompassed. I am sure that when I meet the Warrior and Arctus again, they will have brought similar burdens. My guesses are the Lord and Ritual, but I do not know and would be delighted to be wrong.
"Naturally, many of my acolytes and lovers and assassins want to come with me, and I will not tell them no; the way to the Ruby Throne is long, and I have not taken so long a physical sojourn as this since lending my hands to the Ghostgate. I will enjoy their company and answer their questions, too, and no doubt I will put many to bed with song with the borrowed voice of my sister. Further still, my coming is known and the manner of my coming, and I understand that there are lorekeepers that want my counsel, my explanations, the light of my heaven-seen face. I sent warnings that no riddles will come of it. The lawyers are pouting now.
"Understand that I am a Master and make my own way. For me, Mystery is dead. That is so hard to write. You cannot feel the pause. My brother is dead.
"So for all these reasons and more, and mostly because there are more mysteries in the Capital that I do not know and shall take the place of those I own, this pilgrimage shall be filled with nothing but untangled truth about the Tower. I can hear him now: what of Auriel's breaking? What of my twinned antecedent? What is the meaning of the endeavor that Artaeum refuses to admit? What, indeed, said the Thief to the Master that mostly knew.
"For my part, I know my own first question: What do you want to hear first?"
--Book of the Last Hour, Vehk and Vehk