Tim Harding has written a book Four Gambits To Beat The French, Chess Digest, 1998, covering the Alapin French, Winckelmann-Reimer Gambit, Wing Gambit and Milner-Barry Gambit in detail. As one of the "lesser" gambits, Tim briefly deals with the DDG in three pages, presenting three DDG games: Duhm-Martin (1908-1909), Diemer-Buerger (1948), Heikkinen-Watson (1996). He mentions the DDG pages.
Tim Sawyer has published The Blackmar-Diemer Gambit Keybook II (Pickard & Son, 1999), and the theory of the BDG is stronger than ever. In the annotations of the only Alapin-Diemer Gambit game of the book, there is one nice game transposing to the DDG: Tom Purser - guest, ICC, 1997. When are we going to see such a book on the DDG?
Jan Erik Zimmermann published my DDG article in K-skakbladet, a Danish correspondence chess magazine. The article just rehashed two analyzed games on the DDG Keres Variation.
Eric Schiller has published some analysis on the Tarrasch Defense - Marshall Gambit [D32] 1.d4 d5 2.c4 e6 3.Nc3 c5 4.cxd5 exd5 5.e4 dxe4, which can be reached in the DDG, e.g., via 1.d4 d5 2.c4 e6 3.e4 c5 4.cxd5 exd5 5.Nc3 dxe4.
Eric Schiller responded via e-mail to the article Schiller on the Keres Variation, DDG News 1/1999. He agreed that nasty 15.Nfd2 holds up, and suggested 12...Re8 (instead of 12...a6) as an improvement. He will give the Keres Variation a more thorough examination in a forthcoming book Refutations and Remedies and will also post the relevant analysis at Chess City. DDG News will then, naturally, cover these new analyses.
Gordon Evans recommended the DDG for IECG (International E-mail Chess Group) members in WebWatch, the IECG newsletter: "It does contain some venom."