Hi,
In any other (German) version of the game (Steam, DVD, ...), a PDF version of the rulebook for the DSA (The Dark Eye RPG) is included. But I could not find it here. Is it not included at all? Why is that no mentioned anywhere?

I think that was only included in the "Drakensang Gold" edition that was only released in Germany as retail.

This response is probably way too late to help you and I haven't installed my GOG version yet since I still have my retail version installed. I'll have to switch it out one day, but currently playing Dragon's Dogma DA and that game is such distracting fun. :)

But I think my retail version has the manual in the game folder (or I could be wrong and thinking of another game), so may want to check there just in case. I apologize if I'm wrong though. =P

Often games do that or sometimes it's part of the in-game help menu, so it's always worth checking both. This tends to be the case when you don't see any manuals in the "Extras" section. At least that's been my experience with the exception of a couple of games.
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There is a manual in the Drakensang directory. Here is a screenshot:

Thanks for posting that, I'm sure it'll help others in the future.

I really wish GOG would at least put a description in either the game store page or game download page to avoid having people wonder. It seems like such a simple matter if done during the initial release of the game.
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NP :) I try to be helpful. Doesn't always work, but I try.

The book The Dark Eye Basic Rules (a 290-page pdf of the hardcover edition 2008) is not part of the GOG version. Only the manual is included in the games directory after installation.

Nope, it was inlcuded in every edition. Or at least in the original release and the Gold edition, as well as on the River of Time / Drakensang bundle.

I never received a copy when I bought Drakensang off Gamersgate back at launch.

Not sure if you meant boxed copies or what, but I can assure you I have never had a copy of that rulebook, and I bought every game as soon as it came out.

You're right. I meant boxed copies. I need to install a DVD drive, then I can copy it from the disc.