Actually you don't need to run back out of the room. You can read the book right there and the dragon will transform. 1 Hide in Shadows should be sufficient to run up.
Also, try standing on right (or left but not center) of the green mist and facing it to pick up the scale.
Thirdly, Amdusias seems to die relatively easily in melee combat, especially if you don't have a very high damage dealing ranged weapon.
