I'll say it right off the bat : "Never Unprepared" is not the book you are looking for if you want to actually get better at preparing your games. Even if there are some useful things buried in it, it's mostly hollow talk and useless advice. This comes as a big (and bad) surprise to me : Eureka and Masks are both very good products that you should buy, Gnome Stew is a very interesting blog that you should read, and posts from NU author Phil Vecchione about prep are really a must. Here is my more detailed review.
The book is 131 pages long, with a very complete index and a table of content, so you get about 120 pages of material. It is divided in 14 chapters, one of them being the conclusion and another one listing references and inspiration, plus a foreword, an introduction and a "how to use this book" passage. Quibble here : introduction and conclusion should be treated the same, so both a chapter or none, while references and inspiration should not be a chapter. The main 12 chapters are divided in three sections, and I'll treat each of them separately here.
The first section, "Understanding Prep", contains seven chapters and fills about half of the page count. Another quibble : the first two chapters feel like an introduction, and maybe should have been lumped together into one. In chapter 1 (Prep is Not a Four-letter Word), the author explains why he believes prep has a bad reputation, and why it should not be this way. In chapter 2 (The Phases of Prep), he gives an overview of the five phases of prep that will each get its own chapter : brainstorming (sparking ideas, chap. 3), selection (choosing some ideas, chap. 4), conceptualization (expanding and fleshing out ideas, chap. 5), documentation (making actual notes, chap. 6) and review (making sure you did not make any mistakes, chap 7).
I could go into details of each of those chapters, but it would be repetitive. They all takes a lot of time explaining what is the phase, why it is important, what problems could happen if you do too much or not enough of it, but does not give much actual advice to accomplish it successfully. There is a "Techniques for Improvement" subsection in each of them, but they're all useless. For example, the three given in the Documentation chapter are really nothing more than "think before you write", "don't write things you don't need" and "make sure you are comfortable with your pen or computer software".
I also have issues with the division of phases itself. It does not strike me as the best one to help people understand and get better at prep. The author insists a lot about the fact that conceptualization and documentation are really distinct phases (the first being the thinking, the second the note-taking), but it strikes me as mostly a matter of semantics. I mean, yes, you can distinguish the act of thinking from the act of writing things down, but in the spirit of getting better at prep, a much more productive distinction, in my opinion, would have been to talk about first preparing the general outline of a session/story arc/campaign, then preparing individual scenes in more details. Reading the book, I am under the impression that the author actually sometimes confuses his own phases with these.
The second section, "Prep Toolbox", contains two chapters : chapter 8 (Tools for Prep) and 9 (Mastering Your Creative Cycle). I thought the section would provide tools to prep my sessions, like templates, plot flowcharts and whatnot to use and hack to fit my needs, and boy, was I wrong. Instead of that, the "tools" of chapter 8 are all about the things you use to prep, like notebooks, computer and pens. I kid you not, there is a table listing pros and cons of pen and paper vs computer, stuff like "paper tools don't require power" but "can't capture audio and video". If the chapter was talking about how to get most of different online tools (here are some great generators, here is how to use Obsidian Portal, etc.) or how to prepare material for your games (draw some battle mats, write conditions on index cards, etc.), that could have been acceptable. But no, there is not a single word on that. What you get instead is stuff like "If you use a notebook, you shouldn’t have to worry about the pages falling out" (p. 68).
Chapter 9 comes down to "make a schedule and plan some time for prep", and seriously blow this thing out of proportion by advising to also plan how your creative energy cycle on a hour-by-hour basis using a 0-to-3 scale (with a color-coded table and graph).
I could not believe I was reading those two chapters in a book devoted to prep, especially since the author repeats many times in other chapters that feeling comfortable using your notebook or software or whatever is important. Think of it this way : of all the things that could have been done in a section titled "Prep Toolbox", NU chooses to elaborate on things third-graders are told on their first day of class. Seriously, this section is so ridiculously inane it's almost insulting.
The last section, "Evolving your style", contains three chapters : chapter 10 (Your Personal Prep Templates), 11 (The Prep-lite Approach), and 12 (Prep in the Real World). Even if it is far from perfect, this is the most useful section.
Chapter 10 opens with this line : "Up to now I’ve avoided talking about what actually goes into your session notes" (p. 86). That hints at how useful the first 9 chapters were. This one gives you some actual usable advice about how you should organize your notes to make them more effective and useful, things like a list of "a list of common GMing weaknesses and some ways to compensate for them" (p. 90). Even if the ratio of good stuff/useless stuff is better here, there is still a lot of filler, like the first 4 pages of the chapter that are repeating things you read before, the "Paper vs Digitial" subsection (that again?) and unfortunately the whole "Template Maintenance" subsection, supposed to give you tips on improving your templates, that just gives you hollow tips of the "if it is too long, make it shorter" kind. It's the best chapter of the book, but I would give it a 3 out of 5 note at best.
Chapter 11 gives you some advice to actually reduce the prep you do, like ways to simplify the stats of your NPCs or to make maps way faster. Even if the presentation is incomplete, they are interesting and useful ideas that you can actually use to prep faster (still, like in the previous chapter, they are swimming in filler). You should know that the author already wrote a series of posts about it on Gnome Stew. The good stuff in this chapter is pretty much directly lifted from it, and there is actually more in the blog posts than in the chapter. NU actually tells you that if you want more details (about all these things that could actually improve you prep), you should go read them.
Chapter 12 gives advices on how to deal with problems that are all variation on either "I need to prep something really fast" or "I want to remove something from my scenario". The way to handle them comes down to "cut down on some phases of prep" and "go back to some phase of prep". There is also the "I was planning to prep my game Thursday, but some other thing came up" problem that meets the "plan some other time to prep" solution. It's a useless chapter.
In my opinion, all the problems of the book comes down to the fact that there is a lot of space dedicated either to explaining and analyzing (and repeating) stuff that really does not need much of it, or to deliver advice that are self-evident and begs the question. For example, the whole chapter 4 is basically only advising you to know your players, your game, your campaign and yourself as a GM. Unless things like "some players love a long dungeon crawl while others want to play out trade negotiations" and "having space aliens invade your Dragonlance® campaign in flying saucers is likely to cause a disruption at your table" are eye-openers to you, the elaboration of those four elements is pointless. To help you with those tasks, there is nothing aside providing really general questions that you probably already ask yourself ("what are your favorite parts of a session?") or don't even bother to because they'll come naturally to your mind when needed ("does your game feature a social combat system?").
Among all that, you will find some good things in the book, but they often feel very incomplete. The reason is that the good stuff is usually only there to illustrate some truism like "your notes should be well-organized" instead of being presented and detailed as a tool in itself. Here is an example taken from the Brainstorming chapter : "What kind of session do I want to have? (As in a chase, a rolling fight, a heist, etc.)" (p. 22). Someone in desperate need of ideas gets a lot more help from reading "a chase, a rolling fight, a heist" (and would get even more if he could read all these words folded in this "etc.") than from being told to ask himself the suggested question. Some chapters are less bad than others, but this is characteristic of NU as a whole, and it's very irritating.
You'll also find some usable but really bad advice. The worst offender is probably on page 44, where it discusses the eternal "A problem has an infinite number of solutions, but your players will only ever pick one" issue. NU's answer? "It’s better to expend energy on the most likely solution, plus perhaps the two next most likely contingencies if you have time." It's like saying that if you prep well (and more than you'll use), you will never be surprised by what players will throw at you. Of course, that is very false. It could be argued that preparing for those situations is exactly what prep is all about, and NU does not even seem to acknowledge that they happen.
Bottom line is that any book that wants to be "The Complete Game Master's Guide to Session Prep" is going to face the problem that not everyone has the same issues with his prep and does not want to achieve the same things with his game. A good one would feel like a knowledgeable worker walking you down the aisles of a big tool store, telling you how to use each of them so you can make an informed decision about which ones you'll bring back home. "Never Unprepared" feels much more like this knowledgeable worker sits in your living room, telling you that you should really go to that tool store and choose stuff you will need, weighting the pros and cons of using a handbasket or a cart to shop and, sometimes, letting you catch a glimpse of some shiny things that he brought from his own toolbox.