I think two different books would be a good idea, one to increase the RATE (but not the NUMBER) of elite zombies on missions and another to decrease the rate. I would also suggest that the book affect both rate and quality (chance of a unique vs a rare).
It sounds like the elite zombies are fairly well balanced now, but people still have different opinions about how much fun they are. While we all get up to 3 a day, it would be nice if they showed up more quickly for those of us that like to hunt them. Conversely, those that don't like them might want to make it less likely they'd show up on a given mission. Books that increase or decrease the spawn rate of elite zombies could help with this.
It seems to make most sense to have timed versions as a rare book drop in loot. A permanent version would probably need to have a short to non-existent slot lockout, but would be valuable to buy for fuel in the store.
Since unique elites are worth more fuel than rares, if the book also made it more likely to meet uniques it would increase in fuel value to players. Additionally, those who don't like elites could hope to face weaker ones if one did spawn.
I would probably buy such a book. Would you?