I just finished his newly revised and expanded edition of Searching for God Knows What (compliments of book sneeze),and found it to be a powerful and inspiring resource for spiritual growth.
Miller says that theology is primarily relational rather than propositional. If we don't have it right relationally, then we don't have it right -- even if we "have it right." In his words, "Being a Christian is more like falling in love than understanding a series of ideas." It's a narrative, not a formula.
He makes his point vividly through excellent story telling and colorful imagery. For instance, as he describes what he imagines the Garden of Eden was like, I almost felt like I was there!
One special perk with this edition - -there's a cool little secret code search game embedded in the book with a nifty decoder inside the back cover. You can receive clues by going to http://www.donaldmillerfan.com/