NOTE: No more placeholders and the respawn time is unknown but is greater than 13 minutes. We gave up after 20 minutes and bought it in the broker.
UPDATE: can farm the book from Agony multiple times by only killing Agony and NOT Doom or Despair. You can lose aggro by running and zoning in to Cazic Thule or by using Feign Death. You then need to wait only 5 minutes for a respawn of Agony.
Spawns near the lake at the far end of The Feerrott near the Cazic-Thule entrance. Groups of wraiths are PHs. It's approx. 13 min between PH spawns.