The best way to get to Maar Gan is to take a Silt Strider there from Ald'ruhn. Once in Maar Gan go to the Tradehouse, which is north of the Silt Strider ramp. Withershins is in a locked (70) crate by the desk.
There is also a copy of Withershins in the Ald'ruhn Mages Guild. The book is on the shelf behind the imperial cult altar. Alternatively, the fastest way to finish this quest is to pick it up on the first quest for Aengoth the Jeweler. Keep this copy until speaking to Aengoth about Withershins. When asking about Withershins, he will ask if the Nerevarine has it. Say yes.
|10||Aengoth the Jeweler asked me to bring him a copy of the book "Withershins." He thinks Miles Gloriosus in the Maar Gan Tradehouse has a copy.
|100||Aengoth the Jeweler thanked me for bringing him a copy of "Withershins".