It's the base sourcecode of the game which you won't find in your addons.
But you need to check the lowest lines to see how the error occured and moves into the base code then:
->/AddOns/Librarian/Librarian.lua:563
Something is not correct in the code where the pressed mouse button is released above a row of the Librarian list.
Code:
user:/AddOns/Librarian/Librarian.lua:516: in function 'Librarian:ReadBook'
user:/AddOns/Librarian/Librarian.lua:563: in function 'LibrarianRow_OnMouseUp'
LibrarianFrameList1Row1_MouseUp:3: in function '(main chunk)'