Unlike other books on weight loss, this book is not about a specific diet, but rather everything that revolves around the actual dieting. There are a few basic principles that are common in all diet setups, he argues, and the reason why diets fail is because the side effects of the diets are not properly addressed.
For example, the basic idea behind weight loss is to create a negative energy balance such that the body is forced to draw upon its fat stores for energy. There are individual variations on how to best achieve that effect, depending on many factors, such as age, sex and insulin sensitivity. For someone with pre-diabetes (or worse), lowering the carbohydrate intake (specifically high-glycemic) is a good idea, but for others it might not matter where energy is cut, as long as the essentials are provided for.
Who Is The Book For?
Now, being on a diet - in most setups - is about depriving yourself of something, be it carbs, fat or just energy itself. What happens if you by accident deviate from your diet plan? Will that one piece of "forbidden food" be enough to satisfy whatever craving you have, or do you start binging uncontrollably? It's the difference between being in control, and leaving control to an outside force: if you put yourself in a mindset where you don't feel guilty about having satisfied your craving, but instead tell yourself it was a planned step out of the path, you will have a higher chance of successfully returning back and following through with your diet. This is a very important realization to make, as it is for many people the make-or-break of a diet. Moreover, you might find yourself in a situation, such as a vacation trip, where it is simply too inconvenient to follow your diet. Lyle here introduces the concept of a "full diet break" with a few eating guidelines on how to make the best of the situation.
Other concepts, known from books such as "Ultimate Diet 2.0" is the structured refeed, performed to raise metabolic state and offset the tarnishing effect of being on a diet for longer periods -- for a large person, it might very well take years to slim down to a healthy weight, and planned breaks or refeeds once in a while can go a very long way towards following through with that entire year (or similarly long period) of dieting.
Finally, the book discusses how to eat at maintenance - you're not going to diet for the rest of your life, are you? While I don't fully agree with Lyle's view in terms of what constitutes healthy food, the guidelines are good enough for most people, and if you've been on a diet for a longer time, chances are you'll be able to adapt his recommendations to suit your specific needs.
Conclusion
Slanted towards the psychological side of dieting, the book is fairly unique in that it does cover the actual dieting part too. Usually, you'll find books addressing either side, but rarely together. It's an interesting read, if you're wondering how you should deal with your diet and have no other material by Lyle. If you have previously read "Ultimate Diet 2.0" and "Rapid Fat Loss Handbook" (or "The Ketogenic Diet"), it does not offer very much that is new, however, but it is still an interesting read.
Lyle's writing in A Guide to Flexible Dieting is clear, informative and casual, making it a quick read, hopefully leaving you with a new perspective on how to go about with dieting and the various aspects involved.