I've personally killed the Spy over 40 times, and have never gotten anything other than the Informer's Necklace. However - I did watch someone fight hime, and the Spy had a large book in his hands. The book was the book of secrets, and the stats were something like -7str +7int -25hps +25mana, plus some resists, Range slot. But judging by the fact i've killed him litterally dozens of times and have never looted this - it must have a drop rate of like 1 in 100, so is CRAZY rare.