The sleeper zombies do get harder the longer the further your gamestage is though. It gets very iffy when you're running into radiated feral sleeper.... especially when its the cop version.
That said, super loot you got there. I had to wait till like Day 12 to get my mini-bike book... Raided 2 crack a book buildings, 1 normal book store, 2 day care centers, 3 shanty town book stores and I did not get a single mini-bike book.... got it from looting a random house instead..... on the upside, i probably have enough paper to cover my shotgun shell needs till the next alpha (^_^")