Ok, so I'm not sure if this is really the original notebook, but I expect it was one of the earliest.
In the above photo you can see that I drew a smiley face and that I wrote the name of my home state.
I discovered this in the SCA tents at the Skagit Valley Highland Games about a month ago, and I got one of the SCA people to tell me about it.
The "book" is made up of three pieces of wood with wax on the inside sides of them. A person can take notes or compose a letter in this notebook, and then transfer what they've written to a scroll or paper. This had two advantages that I was told of:
1) A person could draft a letter on the wax before writing it down on expensive paper.
2) If a doctor-type person wanted to take notes on herbs found in the forest s/he could use this rather than taking an expensive book (which would be expensive because they couldn't be mass produced like they are these days) into the field and risk damaging it.
Once the notebook is full a person can hold it so that a flame heats the wax and smooths it out. Then -- ta da! -- the notebook can be reused. :)