You can rebind a book if the old binding breaks down or is not to your taste anymore. It does not have to be the original book either. Someone might have made a copy to preserve the text. But copies create errors too.
Hadn't even considered rebinding. I'd have to know more about the history of binding to know if a pre-leather binding bound book could be rebound with leather.
If anyone is interested here is a link to a list of the worlds oldest books
Yes, please. Check out the Celtic Psalter, which is in Latin using, roughly speaking, our alphabet as an example of how incomprehensible the text likely is even if written in a language Harry is familiar with. I could catch a few words. The Nag Hammadi Library shows that my first source had leather bound books about 100 years younger than they are. It also shows what a surviving leather bound book from Merlin's day would look like without some miraculous preservation.
Merlin's secrets are probably too "sensitive" for broad dissemination. Like if DIY $50 WMD's were widely known.
Yes, but if the information is worth writing down and preserving, one has to balance risks of dissemination with risks of the info being lost.
A lot of the problems with errors caused by copying and dissemination could be solved if the copying was done through magical means.