I'd like to see more game guide books as extras. Many old games had great player's guides that had all sorts of extra information, plus some of these can be hard to find otherwise. Some old games also have fairly lackluster online guides.

I'm guessing that there might be some sort of licencing issue with this, but if it can be accomplished it would be a great feature to add.

The problem would be that the guides are property of a company like Prima, so they would have to negotiate a deal with the publisher of the guide.
Not impossible, but probably not worth the cost and effort for GOG.
I wouldn't mind some cool guides though, some of them are almost art.

The Longest Journey has one. Made by the GOG team, I assume.

I think the guides used to be usually made by the same companies that made the games themselves, so I don't think it's such an issue with older games.