Here are some tips on sourcing books and technical manuals for your G
1. GwagenPreserve.com has high quality english DVDs available of all the factory service manuals in high resolution including photos
2. Books4Cars.com in Seattle always has a large collection of Mercedes books, manuals, etc including G wagen manuals.
Currently they have 1483 Mercedes manuals for sale. Some are originals, some are just copies, so make sure before you order on what you are getting.
3. Mercedes Europe Classic Accessories
This site I have been ordering from for many many years. It has an ever changing list (changes weekly) of brand new old stock items, accessories, books, clothes, brochures, posters, even stuff back to the 1950s that they find who knows where in the old mercedes buildings and they put it up for sale.