I didn't follow everything too closely, given the amount of freetime I am currently having, so I just wanted to ask:

Do we know when and where we will be able to download the extra digital contents like the CP 2020 sourcebook?

Thanks in advance!

At least not until the game is released: 16 April 2020, if there's no delay. If you preordered it on GOG, you can download them under Extras/Goodies when the time comes.

Ah, ok, assumed as much, but thanks for confirming!

Much obliged!

Not entirely sure, but I think you could download some tracks of The Witcher 3's soundtrack prior to its release/pre-download phase.
Maybe CDPR will do something similar with Cyberpunk 2077, to further incentivise buying the game on GoG.

The wishlisted GoG-exclusive demo could still happen as well:
https://www.gog.com/wishlist/games/cyberpunk_2077_demo_via_gogcom