Hey there

So I bought R:RC and digital artbook DLC I download it, then install it and can't seem to find it. Neither in game folder nor game itself. What am I missing?

This is because GOG has a tendency to create installers that actually don't do anything at all. This happens with many games, and eventually they simply remove the .exe file. What you want to do is go to your library, navigate to the game, expand the offline installers section, and look on the righthand side. Instead of an installer is a zip file containing the artbook .pdf under the Extras heading

I was wondering about that myself. Thanks for the heads up.

It's an ongoing problem and has been for quite a while. You almost have to have a basic knowledge of DOS to figure out where your files are from GOG ;)