Some firearms collectors summerized a book out talking about "misc" stuffs in gun collecting. It covers many topics, one topic is regarding gun restoration. Although the book was written mainly by collectors of Winchester, Colt, etc, I feel this book is generic enough about this hobby's context -- It does not cover any German guns in detail (probably does not cover American guns' detail either but I don't know those), it serves as a high level guideline, and serves that purpose well, IMO.
The book is readily available from Amazon.com for five bucks.
Link: The Gun Digest Book Of Firearms, Fakes And Reproductions (Paperback)