I love both books. Desigining Great Beers gives you the background to be able to brew just about everything once you understand the first part. The second part, where he examines the particular styles, is the best part of the book, agreed. It's unfortunate he doesn't delve into every style in the BJCP, but since he covers most of the major ones, you're gonna be able to extrapolate into whatever isn't covered.
Brewing Classic Styles (BCS) is also good, I really like that it covers all of the 80 substyles in the BJCP. It doesn't give the entire background of each style like DGB does, because it's primarily a recipe book. And though the main recipe in each chapter is for extract brewers, he has an AG option included for each recipe. The other thing is that Jamil has strong opinions on some of the styles, and it comes out. For example, in the chapter on Classic Rauchbier, he says twice, "Do not use peat smoked malt under any circumstances." I just find that funny.
Finally, BCS cites DGB as a resource in the opening introduction. They're very complimentary books.
I assume you're familiar with the "Classic Beer Styles Series"
that covers 10 of the particular styles in-depth?