I can't answer that question with any authority, but I would imagine that keeping a log of what one has done is critical - restorers may work on priceless artifacts, and so need to be able to justify the choices they've made. Plus, if a restored piece is sold to a museum or via auction/through a dealer, those records would be part of the provenance & a useful guide for any future restoration.

If you have specific questions, I can get them answered - I have a friend who's an art dealer in Florence, and likely keeps up with this field although his specialization (renaissance drawings) is fairly narrow. But I don't know if I can ask him for a primer without knowing more closely what period you're looking at! (And keep in mind that the field has advanced to the point that one of the tools for analysis is now Raman spectroscopy, which analyses the molecular structure of pigments... Maybe start with Dark Water? It'll give you a great look at what kinds of damage can occur and a sense of the challenges the field faces, which might be a good place to start...)