pretty cool idea. I didn't know at first what the books were, and just picked them up and then looked at my inventory and went like.. "oh no.."
At first the effects seemed subtle, but then it suddenly got insanely hard (around when I picked up the second agro book, I think).
The controls could use some work, though. Would have liked to be able to shoot diagonally (or at least that the latest pressed key took precedence), and the enemies weren't terribly interesting. And there seemed to be some reload or something? the gun stopped shooting occasionally, but without a sound or anything to indicate it, it was a little annoying to keep track of.