Each chapter in this book -- some brief, some longer, as the material requires -- discusses the various positions surrounding one particular aspect of the subject. For example, there are several chapters about whether a kli sheini cooks its contents, depending on whether they are solid, water, or other liquids. Each chapter is backed up by extensive source material in Hebrew in the back of the book. Many of the issues are as old as the Mishna, but some are as new as modern technology can make them. (Is there a problem if your hot-water urn has a glass sight-tube? Can you reheat food on Shabbat if the food was originally cooked in a microwave oven, which is not halachically fire?)
The genius of this book is that Rabbi Willig explores the underlying halcahic constructs that inform the piskei halacha that the sources bring. This of course is how the Talmud works: given opposing sets of halachic decisions by some number of rabbis, the author tries to deduce, from the shape of those decisions, how each decisor views the halachic and physical realities involved. It is a joy to see Rabbi Willig propose test cases and tease apart the underlying structures.
In many cases he does not even provide a definitive answer, since there is room for different rabbis to interpret the halacha differently, and one should be asking one's rav, not a book.
In other words, this is not a "quick reference" book that gives quick (and safe) answers for the lay Jew. That ground has been masterfully covered by the Shemirat Shabbat ke-Hilchata. Instead, what Rabbi Willig has provided us is much more profound: an insight into how the different schools of thought understand the nature of Shabbat, the nature of food and food preparation, the nature of halachic divisions into the permitted and the prohibited, and how these spheres intersect.
It is clear that this book is adapted from shiurim that Rabbi Willig has given at REITS. In fact, my main frustration with the book is that it's not always clear when a cited source is going to be quoted in full in the back of the book; I feel like the material is intended to be followed the way it would be in a shiur, with a source packet in hand and with the reader's attention being directed first to the original source, then to the maggid shiur's explanation of it, then to the next source, and so on.
I soon figured out that the right way to read this book is to read a chapter, then read the sources, then came back and read the chapter again. Each chapter does end with an clear and brief summary of the issues considered in the chapter and what the major opinions are on them. That helps, but not as much as integrating the source material with the main text would have.
And I'm afraid I must disagree with the series title: This is not a book about practical halakha. While I am glad to see that Koren/Maggid/REITS/YU sees a need for a more centrist alternative to the more reflexively machmir books that are out there, this does not address that audience. A pashut yid looking for a practical guide to the laws of cooking and warming food on Shabbat will not find their questions answered here.
To stress: That is not Rabbi Willing's fault, and it is not a fault in this book; it is a mismatch between what this book is and how it is being positioned in the marketplace of ideas.
But for those with an interest in the halachic process and in gaining a deeper understanding of how the halachic mind grapples with questions that we take for granted week in and week out, this is an excellent book worthy of space on your shelves and in your thoughts.