Wabbajack (author unknown) is the story of one man’s summoning of Sheogorath and his ensuing descent into madness. One suspects the journal was penned well after the fact.
It can be found in:
The College of Winterhold, Arcanaeum, on a ledge.
Lost Knife Hideout, on a bookshelf just before the cages.
Reading by Galev (@galev_ph)