[MSEA] Time Temple and KoC

Hmmm why was that exactly? Does it drop somewhere?
Or is it the lvl120 Thief boots that give the +1 Assassinate so you can reach lvl 21 Assassinate while having lvl 20 Assassinate?

They drop from Guardian Captains of Oblivion (The dark-red colored knights), which are hunted a lot. Also, everyone who wants to unlock all Time Temple maps have to eventually kill 999 of those, so a ton of the book is bound to drop.