Two Phase Book

"Voila!"
The cherubic shop owner dropped two books down onto the counter with a satisfying thud. The two books were identical. Both were about an inch thick from cover to cover. Both were bound with a fine soft leather dyed blue. Both were bound and wrapped with a very fine silken thread. Each had a sigil pressed into the cover. Finally bordered by a series of arcane runes.
"One two phase book right here"
The plump shop owner punctuated her statement by poking one finger on each hand onto separate books. The patron blinked in apparent confusion at the statement trying to understand if he heard incorrectly. Clearly, there were two books in front of him. How could there be only one book? he thought. Seeing the confusion the shop owner explained.
"Now don't worry, I am not trying to scam you. It actually is only one book. Enchanted it myself!" With a grin of pride she continued.
"You see it's actually rather brilliant. If you make two books and bind them together, you have to then add all sorts of enchantments. An enchantment to detect any changes made to one book. An enchantment to "send" the changes to the other book. And finally an enchantment to the create the materials for the changes and apply them exactly to the other book. Then do that for both books! Not to mention you have to be on the same plane for it to work properly. It really is quite a bit of work and much harder to do. Once you start stacking enchantments..."
With a feigned look of exasperation the shop keep rolled her eyes and looked away. It was all part of the sale, she thought. Now to wow them.
"Instead it is much easier to trick a book into thinking it is in two places at once!"
The dumbfounded expression of her customer told her she was on the right track.
"Much simpler. By making the book believe it exists in two places at one, it practically does. When you write in one book the 'other' book follows suit. Even across planes. After all, why not? It's the same book!"
As if it truly were that simple of a task she continued, "Perfect for pen pals, long distance lovers, or even..." Her voice lowered to a whisper and she leaned in close. "...if you needed to get a quick bit of intel back to your higher ups."
Springing back upright with a hearty laugh she clapped her hands down onto the counter top. "Not that I would openly supply discrete versions of the same thing!" She threw a wink at her customer. "Now come on, let's talk price."

Mechanics & Inner Workings

Any changes made to one book immediately happens to the other book. Any writing is instantly copied over. If one book catches on fire the other will as well and burn in the exact same manner. (Subsequently putting one out will put them both out.)
If a page is ever torn out, both pages actually reduce to dust. Do to the nature of the magic when removed from the binding half of the page goes to each location.

Manufacturing process

After creation, one book is enchanted to become two books. In layman's terms the book now exists is two places at one.

Item type

Book / Document

Rarity

Uncommon

Raw materials & Components

Typically made similar to many other books. Pages wrapped in leather and bound by twine or glued.