Vade Mecum were medieval notebooks. They hung from belts and were used by doctors to carry important reference materials with them when visiting the sick. I've made this one inspired by the style but using individual pages for a notebook. Not very authentic.
EDIT: I've now found evidence for this sort of loose leather cover. So there are really only two things about this not authentic. The individual pages and the button to keep it closed. So, about 80%. That's pretty good. I think I will make some of these for Spring War.