You're spot on with how reusing materials gives a book its own story. Honestly, it's what makes handmade journals so special. Tbh, using old maps like that adds a layer of history you just can't buy new. Each crease and faded spot has a past life, and now it's part of your writing. That kind of character makes you want to pick it up and use it more. Ngl, it beats a plain store bought notebook every time.
Try sealing those map pages with a clear matte spray fixative. It keeps the paper from getting more brittle and stops ink from bleeding through the old print. I learned that after ruining a cool blueprint page with a fountain pen. A light coat lets you keep the look and feel without messing it up as you write.