In this book, Dempsey proves himself to be one of the most intelligent boxers the ring has seen. His knowledge of the physics behind a punch is impeccable, and his book is based around using these elements to your advantage. The best example is bodyweight, and Dempsey describes in the opening chapters how a baby dropped from a 10 story building onto a person's head can easily kill a grown man. You've got 20 times a baby's size - imagine what you can do!
The rest of the book details how to throw pure punches, with body weight in motion behind each to ensure maximium damage. He knows quite a bit about preventing punches from losing their power by keeping skeletal structure optimal, and throughly discusses this in the chapters about the power line.
In addition, this book isn't just about boxing. It is written with fist fighting in mind, where K.O.s are much more prevalent. In these situations, being a "fancy Dan" is begging for a knockout. Each second you're fighting exposes you to more danger, and so ending the fight via power punching is the best exit. And boy does he show you how to do it.
A side note - the basics of a punch where the fist lands vertically isn't known by many modern fighters. This punch predominated in bareknuckle boxing because it allows for a cleaner strike with more of the fist, whereas horizontal punches tend to connect in smaller areas and increase the likelyhood of breaking a knuckle. When you consider the pinky is most skeletally reinforced part of the fist, a fact Dempsey explains in the book, this is even more dangerous (he teaches you to aim with the ring to prevent pinky breakage while still allowing for high power). He doesn't mention this protective benefit of vertical punching, but he clearly knows how to throw them. Then again, this may be due to the fact gloves were around 6 ounces in his day, so they're not quite as protective as they are now. Horizontal punching is somewhat an artifact of the sport, and since Dempsey grew up fighting people in bars, you get to see an entirely new vertical punch most people won't expect. He calls it a straight jolt, and it's like running into someone with your fist. It's faster than my jabs but with serious stunning power.
This book is great for anyone who wants to learn how to punch in a fistfight, as well as a boxing ring. The vertical punch and Dempsey's falling step were both admired by Bruce Lee and incorporated heavily into his Jeet Kun Do system. A cursory look at this book and that martial art easily shows that Lee's ideas about counter-punching were heavily modeled after Dempsey, as with his basics of throwing a punch. And for good reason - if you spend a week on the heavy bag trying Dempsey's advice, you'll notice remarkable gains in power. After a year, you'll be vicious.
He also goes over training and conditioning, including sparring, and the proper way to watch a boxing match. He says he wrote 384 pages for the book then trimmed down the worthless info to the essentials behind strong punches. In addition to having good advice, it's a pleasure to read. The book is well-written, interesting, and gives many insights into how Dempsey thought about boxing (and the popularity of it after his fight with Willard).
When I started taking boxing lessons, I had drilled from this book and studied fights for a year. Apparently this book helped me punch hard from the beginning. I managed to start with few corrections to my general punching form, and since those minor adjustments (the first lesson) I haven't had to spend additional time on basics. The second lesson we moved straight to advanced combinations (the first was more getting me used to the idea of fighting someone). It's very useful to start boxing knowing how to triple jab. And thanks to this book and long arms, I'm told I have good power in either hand.
When I first met my trainer for a lesson, he had me, a friend, and someone who had trained MMA for years jab him in the arm. I was told that I hit harder than the MMA fighter (I had 15 pounds on him but this was my first boxing lesson). I had been training on a heavybag and shadowboxing for about a year with this book and others as a reference, and am pretty athletic. So assuming you have some natural strength or athleticism, it is certainly possible to teach yourself fight using this book. However, timing, distance, accuracy, and defense are hard to learn on your own. But you can develop a head start; by practicing parries and glance-offs as described by Dempsey, I apparently started out tricky to hit (usually new people are punching bags).
Books that are excellent for complimenting this are "The Art of Boxing" by George Carpentier. It goes over snapping punches and more. Carpentier was friends with Dempsey after their heavyweight title match, which was one of the original superfights and million dollar gates (in the 1920s)! It's clear Dempsey organized his defensive chapter based on conversations with Carpentier, who uses the same categories for defense as Dempsey.
The best thing about these books are the little tricks they throw in that are valuable for advanced fighters. They casually mention weakpoints of certain builds, styles, and dirty shots you can use outside of the ring. Dempsey's book, Carpentier's book, Tunney's autobiography "A Man Must Fight," and other Dempsey biographies are chalk full of these references. Find all of them you can and study them - all three were amazing world champions (Carpentier fought in around 6 weight classes and Dempsey and Tunney speak for themselves).
That said, this book is my favorite. It's helped me appreciate a sport I'd neglected for years. Reading about Dempsey's life ("Flame of Pure Fire") and this book, it's clear he was a smart guy. He's so friendly you can't help liking him, yet in a boxing ring he was murderous. He walked 20 miles in the desert for a match when he was starting out. He's the real deal as a champion and a gentleman, and my only personal hero.