It’s an older article, but Slate Magazine has a terrific piece about the importance of narration in the quality of audiobooks, from hard boiled fiction to urban sociology. Read Me a Story, Brad Pitt is subtitled “When audiobook casting goes terribly wrong,” and gives examples of the three most common — and easily avoided — mistakes that audiobook publishers make.
I recommend that you always listen to the audio sample provided at Audible.com or the iTunes Store before making a purchase. Audiobooks can be expensive, and mistakes add up to real money fast.