NL is such a complex form of poker, and this book doesn't shy away or take shortcuts. When it discusses preflop play, it dissects it. Reading it is not a problem, understanding it while reading it isn't that hard either. But learning it takes quite awhile, or at least it would have for me. I've put my NL game on hold for awhile, because I'm simply having too much fun playing limit to re-focus right now.
A good example of a tricky situation is, for instance, bet sizing. What factors should you consider when betting a decent, but not monster, hand and your opponent either has a draw (or one of several draws, even) or a weaker made hand, or some portion of the time a stronger hand than you. This general concept alone takes 12 pages of the book (explicitly) in the section of "Fundamentals" and is of course then implicitly discussed throughout the text.
The pot size philosophy was quite an eye-opener for me, too, along with how to consider deep stacks.
It's a really, really good book, but while HoH gives immediate payout - just after skimming those books, I was three times the tournament player I was before - this book requires a whole lot of thinking and practise before the concepts will take root properly and you can start to apply them. Some of them are easier than others of course, but you still need to practise. Or I did.