the book of explosives is in the office chest you can brake it with your pixaxe and you must have the book of explosives to collect the string which is in the room of the workshop room in the left side on a shelf,but like i said you must have the book of explosices in your inventory,to get this bax you i think its in storage but im not sure or underground beneath the storage room,hope i helped.....
and like people said before after you take the string you mix it with the bax than use it on the gun powder and you got a ready string to be attached to the tnt barrel