Shingeki no Kyojin chapter 85 dropped a bombshell of a revelation after Dr. Yeager confirmed on a photo found on the very first page of one of his books that human civilizations do exist outside the walls.
This confirmation further builds more intrigue surrounding the three books found in the doctor's secret compartment. These should contain information vital in their current battle with Zeke. On the other hand, the secrets Eren, Mikasa, Levi and Hanji are about to uncover could also shatter their morale.
So what exactly these books represent? It has to be these three things: the history of the world, origin and nature of the titan, and lastly the coordinate.
-In the universe of Shingeki no Kyojin, the history of the world is arguably the most debatable topic of them all. As a matter of fact, the only historical facts existing are the ones recorded between the creation of the walls more than a 100 years ago and the present day time.
Other than that, the citizens of the walls have not only been deprived but also strictly prohibited to seek accounts of events that happened before the walls were built.
Dr. Yeager's book will provide the true history of the world before the first Reiss King manipulated the memory of the first inhabitants of the wall, answering the question why it was built in the first place.
-The second book is supposed to tell the nature and origin of the titans. There are already several clues presented throughout the series about its true nature. But there's no better person to explain it than Dr. Yeager.
In his second book, it will answer how the first titans were created and their way of reproduction. Perhaps, it will as well provide insights about the varying classification of titans and how titan shifters are able the harness its power without turning into a mindless killing machine permanently.
-Lastly, the third book is about the much-sought power in the series, the Coordinate. Everybody knows by now that it gives its user the ability to control the titans, manipulate the memories of mostly all bloodlines in the wall (except Asians and Ackermans) and inherit the memories of all its previous users.
Usually, only members of the Reiss family can use the Coordinate, but Grisha and Eren managed to use its power in their respective battles. Another mind-boggling question that could be answered in the third book is why the Ape Titan shows the same ability to control and even turn people into titans.
There's an argument that the third book will help Eren use the Coordinate at its full strength or perhaps it will tell the main protagonist's true identity, which is shrouded in mystery up to now.
All these can be answered in the next issue of Shingeki no Kyojin next month. Chapter 86 will be released on Shueisha Shonen Jump Weekly on October 8.