A case study is basically a real-world example of a business service or product being applied, with real results accompanying the facts and background setting.
Customers can interpret the case studies to judge the practical benefits for their own use, so it is of utmost importance to write case studies that are not too specific, unless it is a very targeted niche that is being marketed to, in which case it would be advisable to be as detailed as possible.
Case studies are very powerful, as they detail plan, action, and results. They are believable, help build your reputation, and entice prospective customers in similar situations to take action and get in contact with you. They also help your clients' profile, as the featured clients are usually quite prominently featured. It is really a win-win situation. Your prospects may be encouraged to do business with you on grounds of possibly being featured in a case study on your website or in your newsletter!
What makes an interesting, attractive case study?
For starters, the liberal use of numbers is encouraged to draw the eye. Naturally, people looking for results will first scan for "$130,000 increase" before comprehending "many new clients over the past few weeks." The case studies must be readable - they should speak to your customers on their level, avoiding unnecessary jargon and the like.
Case studies are most effective when containing a quote from the featured client themselves. It could even spell out a valuable tip or two, which will get people to see you as a generous authority figure - exactly where you want to be!