No-limit Holdem Theory and Practice is a very theoretical book, as expected from David Sklansky. It tries to give an insight into what excellent players think about during the play of a hand that good players probably don't.
Sklansky talks about five skills that are needed for success:

All of the above have their own chapter(s) where they're explained and strategies are given on how those skills are applied. I think, though, that most of those chapters fall short of completely analyzing the situations where the skills can be used. However, i think that he does give an introduction to all of them, and requires the reader to think about all the concepts further. I disagree with that approach at least somewhat. In the chapter of adjusting correctly to stack size he does suggest further reading, but even the suggested book (getting started in holdem by ed miller) doesn't go into enough detail in my opinion. But that is to be expected since it's a beginner's book and doesn't try to be complete.

The chapter about manipulating the pot size is almost complete, i think, in making it clear when and how to use the strategies described in the chapter. Even tho it doesn't (try to) give specific advice for every scenario, it does give enough of a thought process to figure out everything that's needed.

Adjusting correctly to Stack Size was, as mentioned previously, not as complete and not complete enough for me. There is a preflop strategy chapter that has pointers for a stack size of 100 bb. I would have definitely liked the same chapter for a short stack and a middle sized stack (assuming that the 100bb advice holds true for stacks even larger). Even still it doesn't give too many specific pointers about how the play changes from one stack size to another, especially postflop.

Winning the Battle of Mistakes is the one chapter that is complete. It does go into detail about how to change your play considering the opponent(s) you're up against and how to exploit his mistakes, while making less significant mistakes yourself.

Reading hands is the chapter that disappointed me the most. Obviously, this is where i have my biggest shortcomings. While they do say that when you bet top pair on both flop and turn, and get called (by a big stack) you're in trouble. What I really wanted to learn was reading hands, and while the obvious was repeated no specific strategies were given about how to really learn.

The last one, Manipulating opponents into playing badly, talks about a few strategies, mostly making obvious but cheap mistakes to get your opponents to make more expensive (or more errors) later. In online games, however; you might be 'advertising' your bad play and 5 hands later be playing against 3 new opponents. So i think it has a lot less value in online games.

As a whole i think the book is very valuable, and really not made for beginners (as it states in the introduction where it suggests you read the Theory of Poker first, which i think is bad advice because that one also is way too theoretical for a beginner). After reading a book about no limit aimed at beginners, and getting some experience, i do think it's probably the one book that will help your game the most. But i also expect having to read it again a few more times (only read it twice so far) to be able to apply the concepts in the book.