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 S�n�chaud's book Emil Diemer (1908-1990), missionnaire des �checs 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: