In order to find the tome, while in the Soul Cairn go to the area where the most northern Keeper on the floating tower is found. Head southeast from the tower and near the barrier is a structure with steps and a small pillar with the tome. The Tome can be missing so just reload a save or use Unrelenting Force on the pedestal it should be on. If it is missing it may be stuck in the geometry and can be knocked loose. If the tome is missing from the pillar it might also be found down right behind the structure itself.