Sarah Hurst's Chess on the Web reviews some 120 chess-related Web sites. The first edition (1999) rated the DDG site as "excellent" (5/5). Of course, in 1997-1998, at the time of writing of the first edition, there were much fewer chess sites than today; besides, the DDG site was actively maintained at that time as most of the content was created in 1996-1997. Understandably, the second edition (2000) lowered the rating to 4/5.
Eric Schiller's Gambit Chess Openings (2002) updates DDG analysis started in Unorthodox Chess Openings (1998). His new ideas are covered in another article.
Franco Pezzi's Gambitingly! 2002 CD gives a good introduction to the wealth of various gambits. It also covers the DDG, including games and articles copied from the DDG site and converted to the ChessBase format.
Dany Sénéchaud's book Emil Diemer (1908-1990), missionnaire des échecs acrobatiques, 3rd limited edition, 264 pages, February 2003, in French, includes also a brief introduction to "Duhm-Diemer" 1.d4 d5 2.e4 c6/e6 3.c4.
DDG analysis had not been updated since 1999, but now I finally did a major update: