Living Greyhawk Redux
Tome of the Ethereal
A book on creatures native to the ethereal plane
This tome goes into great detail on the anatomy and habits of many creatures inhabiting the Ethereal plane. Familiarity with this book grants its owner a +2 bonus on Knowledge (The Planes) checks regarding the Ethereal plane.
Value: 400 gp
Found in a library on some random plane in Endgame.