I think the best AGS example I've seen is the manual from Charlie Foxtrot and the Galaxy of Tomorrow, which I'd consider gets everything right. It has a lot of pictures, nice layout, credits, backstory, sets the tone, and offers some additional facts about people and places in the game that aren't necessary to complete the game but add colour and detail to the world. It may also have a couple of hints in it, it's been a while since I read it. But essentially I think it had everything covered.