Unless you're providing cliff notes of each chapter I don't think it's possible to give away too much information. When I first started marketing my sales letters were always kind of vague and didn't have much information about the actual product in them, when I started testing the effect of packing the sales letter with info the conversion rates went way up. My best sales letters are the ones that have 20+ bullets on "learn how to...." and "I'll show you how to....", obviously you need to find the right balance and come up with a review that gives them a tiny peak but doesn't solve their problem.