This website can give no more than suggestions and own games about the Dark Knight system; for a more intensive study it is therefore recommended to refer to the available chess literature (classical as well as digital).
The two books by Deppe and Kapitaniak are rather collections of games, which should not devalue them, as they date back to the time before chess databases (one was glad if one could find a sufficient number of games for exotic openings).
Keilhack / Schlenker should not be missing in any library of a TDKS supporter: written in a great and entertaining way.
The books by Wisnewski and Schuyler have the greatest theoretical significance for opening theory, whereby Wisnewski, contrary to his book title against 1.d4 does not deal with immediate 1… Nc6, but with the Chigorin Defence (1.d4 d5 2.Nc6). Whoever wants to play 1.d4 Nc6 directly and also wants to play 2.d5 (and this is highly recommended despite Wisnewski’s reservations) must go to Schuyler.
- Georg Deppe „Die Fischer Nimzowitsch Verteidigung 1. e4 Sc6 2. d4“ (Hamburg 1979)
- Thomas Kapitaniak „Nimzovich Defence“ (Nottingham 1982)
- Harald Keilhack, Rainer Schlenker „1. … Sc6! aus allen Lagen“ (Schwieberdingen 1995)
- Christoph Wisnewski „Play 1… Nc6! A complete chess opening repertoire for Black“ (London 2007)
- James Schuyler „The Dark Knight System – a Repertoire with 1… Nc6“ (London 2013)
So far I have not found anything in digital content that TDKS deals with against opening moves other than 1. e4.
- Christian Bauer „Der nonchalante Nimzowitsch – 1.e4 Sc6“ (Chessbase DVD 2019)
- Andrew Martin „Nimzowitsch Defense“ (Foxy DVD Vol. 40 2014 [Original-Video von 1996]))
- Andrew Martin „Nimzowitsch Defence: 1.e4 Nc6“ (Chessbase-DVD 2016)
- Leonid Kritz „Nimzowitsch Defence: 1.e4 Nc6 2.Sf3 f5“ in: Chessbase Magazin 159 (Apr. 2014)
- Rainer Knaak „Nimzowitsch Defence: 1.e4 Nc6 2.d4 d5 3.Sc3 dxe4 4.d5 Sb8“ in: Chessbase Magazin 138 (Okt. 2010)
Amateur player Gunjan Jani (http://gjchess.com/) has produced a whole series of dedicated and entertaining videos on the Dark Knight system, especially the Carnival Gambit (Colorado Gambit) 1.e4 Nc6 2.Nf3 f5