How to get a book-object ID
 

In the Royal Library at Stormwind Keep, there are books on the table.
You can not pick them up and take them with you.
But you can open them and read them in the window ItemTextFrame.



We can retrieve title of book, text and page number from functions:
ItemTextGetItem()
ItemTextGetText()
ItemTextGetPage()

How to get this object ID ?

The only way I can think of is check the tooltip for a mouseover unit when it pops up and query it for a UnitGUID(). This is if the game triggers the tooltip with tooltip:SetUnit() instead of just tooltip:SetText() internally. It does this with some objects, but not all.

You may also check on events such as UPDATE_MOUSEOVER_UNIT.