I think the core book is the best of the lot. The others add some interesting depth, but I tend to gravitate to the early works (First Fantasy Campaign) rather than the d20 or 4E stuff.
Just my own preference. The d20 books are all well done.
Marv / Finarvyn DCC playtester (2011) S&W WhiteBox author (2009) C&C playtester (2003) Builder of the TrollBridge for T&T; Amber Diceless player since 1993 OD&D Player since 1975; Metamorphosis Alpha since 1976
"Don't ask me what you need to hit. Just roll the die and I will let you know!" - Dave Arneson
If I could own only one of the DAB books, it would be the 4e sourcebook "The First Campaign"
Here is why:
1) It distills, clarifies, and in some casses improves information given across several of the earlier d20 books. 2) It was the last book Dave Arneson published and so can be considered the last word on the subject. 3) unlike some of the other sourcebooks, it is lighter on crunch and rules specific things and heavier on information.