The book you need to collect isn't on the ground, it's on the right side of Cas's House and is floating and spinning and has sparkles coming off of it and you can collect it simply by touching it. It's called "Ass Masters [Limited Edition]". If you don't see it, it's possible you already collected it and forgot about it. If you go into the Pause Menu and select either Skills or Inventory, you should see it after you've collected it.
