I ran across a blog post on mystery shopping, which we often get questions about at the library. We have a few books on it, although the major publishers haven't put one out recently. (Mystery Shopping Made Simple (2004), by Newhouse, is the most recent from a well-regarded publisher.)
But the post at WiseBread does a great job of laying out the facts and pointing the reader to reliable web resources. WiseBread is an excellent site, by the way, for personal finance resources. And don't miss the "forums" link at upper right, to get a taste of their online community. Online communities, particularly when they're a part of a well-run, focused website, are excellent resources for getting tips and information.