Added the manuals to my gogmix. I had already added the 1st, but had a problem with replacementdocs with regards to the 2nd, and that's why I hadn't added it till now. Thanks for the direct link to Myst IV's manual.
This was the problem I was having with Myst IV's manual, so thank you for finding a way to overcome it. Now only remains for me to put the direct links instead for the manuals from replacementdocs on my gogmix (edit-done).

Overlord II is the first release of Codemasters after so many years, yet they can't even bother supplying a manual that is already available on Steam:
https://steamcdn-a.akamaihd.net/steam/apps/12810/manuals/Overlord%20II%20-%20Manual%20-%20PC%20-%20US.pdf?t=1516310465

Added to the gogmix. Thanks!

I seem to have missed this. Addeed now.

Forsaken Remastered might not be coming with any manual, but the original version released back in 1998 most definitely did. While the usual suspects (Archive.org and ReplacementDocs) don't seem to have it (anymore?), I still have it downloaded from somewhere, so I'll just post a link to my dropbox:
https://www.dropbox.com/s/f5c5r47y1szrrld/Forsaken.pdf?dl=0

Added to the gogmix. Thanks a lot!

Added.
We're running very low on space. Looks like we might have to move to a new bigger thread soon.

Sometime within the last 6 or so months, THQ Nordic has changed their website domain from nordicgames.at to thqnordic.com. The new site doesn't seem to have any available manuals, while the links from the old one can no longer be accessed. This has affected the following games in your list:

Battle Worlds: Kronos
Costume Quest
ELEX
Ori and the Blind Forest
Stacking
The Vanishing of Ethan Carter

Unfortunately, I only have backups for the English manuals of Battle Worlds - Kronos:

Extended Manual: https://www.dropbox.com/s/6stv9kk5uimb207/Extended%20Manual.pdf
Weapons Manual: https://www.dropbox.com/s/sauzwxdlrbosm1g/Weapons%20Manual.pdf

Also, Red Faction Guerrilla Re-Mars-tered doesn't have a manual, but the original version did:
http://files.replacementdocs.com/Red_Faction_-_Guerilla_-_UK_Manual_-_PC.pdf

Well, there's always archive.org for cases like this (might not work for bigger manuals), i.e. - https://web.archive.org/web/20180608094828/http://www.manuals.nordicgames.at/files/CostumeQuest/CostumeQuest_PC_OnlineManual_EN.pdf

Does it really work on your end? I get a connection error (timeout).

GOG forum broke the clickable link. Copy/pasta does work for me.

Hm, you're right. Thanks for that. It seems that all the manuals are retrievable, after all. And a bit too conveniently, the majority of them seems to have been archived just two months ago.

Just replaced most broken links with the ones from Wayback Machine in my gogmix.
The English Extended and Weapons manual weren't backed up by the Wayback Machine though, so Grargar's dropbox links were a lifesaver there. Thanks Grargar!

Thanks. I'll update those links.

In the meantime, it seems Redneck Rampage is missing the manual for the expansion.
https://www.gog.com/forum/general/does_anyone_have_the_manual_for_redneck_rampage_expansion_suckin_grits

An update here. The manual is also located in the game's installation folder. In addition, the 5th anniversary version of Oblivion came with a digital map that GOG isn't offering. I have uploaded it to my Dropbox account; your call if you want to use it:
https://www.dropbox.com/s/xumlqtb3slwjte8/Oblivion%20Game%20Map.pdf