Now that I've had the opportunity to use this in game, which I'm currently trying to complete the Side Quest in. I really want to know what the Floating Book is for, other than activating the Keepers. Which does not disappear once they have all been Summoned. As well as had time to think about it. I really don't think it's a glitch.
Simply because by now, as well as knowing how Treyarch are. They would have patched it. Not to mention how meticulous you have to be in what you do. For an average player or someone that don't play all the time. It could become quite the challenge.
It also makes the possibility of a Solo E.E./Side Quest seem a lot more plausible. Co-Op at this point almost seems impossible. You would all have to grab the A.S. at the same time (which seems like a glitch). Or spend a ton of points on the box and get very lucky. Being that the Shadow Man has an actual animation and he's not just "Stuck" there "Frozen". Makes it seem to me as if it was coded.
Back to the topic at hand. Another tip would be if you search one Purple Pod, the next time you search you don't have to worry about that one again. The tail will always be in a Purple Pod you have yet to open.