In this big debate of whether the Soldiers Notebook is vendor trash or
quest item, keep something in mind people ...
EVERY piece of loot you find in Norrath is for one of the following;
Food, Water, Skill Trades or Quests.
If you cast "Identify" on the notebook, it shows "magic item".
The one's who are confident it is only vendor trash think that way,
because no one can find any information about it, so you naturally
call it vendor trash because the notebook means nothing to YOU.
NOTHING is vendor trash unless you choose to make it that.
I destroyed 2 emeralds today while hunting, because they meant
nothing to me. Does that mean they are useless vendor trash ?
No they are used in quests and tradeskills, but I didn't need them.
So in that end, the Soldiers Notebook has a purpose. No one knows
(or is sharing) what it is yet.