Let the reader beware.
Anyone who can get a publisher can get a diet book published. No matter what the author says, his or her goal in writing and publishing said book is to make money. Not to "help countless others" or "spare people from a life of shame and illness".
So sorry to sound cynical, but it's true. Weight loss is a BOOMING industry (for those of you who haven't noticed).
Some of the advice in diet books is laughable--eating according to your blood type (as an example) has no basis in science. Some is downright dangerous--any diet book telling you to eat less than 1000 calories a day can result in permanant organ damage when your body burns critical body mass for fuel (this is the same thing that happens to anorexics and people with cancer or other wasting diseases, and it is not good!)
It's not all doom-and-gloom, however. Some diet books provide good information and guidance. Any recent diet book from the Weight Watcher's group is reliable, and books promoting a balance of sound nutrition and lifestyle changes are out there. You have to separate the wheat from the chaff.
Signs that your groundbreaking diet book may not be up to snuff
CSPI's Bonnie Liebman uses her wit to remind us to use our minds before trying to whittle our bodies. Quackwatch.org helps you learn to look at health claims with a critical eye.
Remember, if losing weight were as simple as running down to Borders and grabbing the nearest diet book, we'd all look like the lean ideal. If it sounds too good to be true...well, you know how the rest of that goes.