When I first accessed the digital artbook and comic everything worked fine. When I started to download the offline installer however when I now try to access the artbook and comic book all I get is 'access denied, please sign in with a GOG account that you have Cyberpunk 2077 on'.

I don't get how is it I could access them before installing the game and now I can't.

You have to use the same GOG/Steam/Epic account credentials, username and password, the game has been added to in order to access the artbook and the Comicbook

There is also a How this works If you are still unable to access the content, contact GOG support or CDPR.

Game has finished installing and I can access the artbook and comic again as I was able to before. Just to clarify I was signing in with the right details, it was only when the game was installing it gave me the access denied message.

Same here for XBox. Opened ticket but Gog.com support is not responding. If you research on Google you will find multiple users presenting same issue.