For me, an excerpt will help a lot. Reviews and recommendations are fine, but many of those are based on criteria that don't mean much to me. Just because I liked a well-written book about a guy raising sheep doesn't mean that I'll be interested in any book as long as it has sheep in it (and no, that's not a real example
). Subject matter doesn't play a big role in what I like or don't like, but style does. An excerpt is the quickest and best way to judge that. If I find the style OK but not stellar, I might download a free book and give it a try anyway, but chances are I won't buy another book by the same author.