You get this for collecting all of Cassandra's books. There're six of them; you can see which you have by pressing the menu button, going to Side Cases, and then RB across to Profiler's Memories.
A full list of where they are, as well as all the other collectibles is here:
But the real reason I am writing this is the cheev glitched on me. It didn't pop when I picked up the last book in the Judgement House. I left my X1 on over night, and the next morning, it had popped!