When I went there, a few years ago, visitors were allowed to stay only in one hotel. Also, infamously, one of the presumed hotels is essentially a façade, which is not mentioned (but the hotel is in the cover!).
Since the population is poor, and there is little mobility across the country, I doubt that they have six hotels in a functional sense.
However, this a photography book, right? Well, they did an obscene amount of photoretouching (pretty much like all the modern photography); the saturation is cranked up to 11. I remember a couple of the locations depicted, and they were completely unremarkable in real life.
So, my suspicion is that this book is more of a "staged hotels of Pyongyang" rather than something substantial.
Regarding your question. IMO, it's worth visiting NK (which is Pyongyang, essentially, with brief tours outside the city) for no more than around three days. There's virtually nothing to see after that, especially considering that you must follow the plan.
What I find interesting in NK is to be present in an authoritarian regime. I can't say "to experience", because the experience itself is staged. But even knowing that, I find it interesting. It's also unsettling to know that you're in their hands.
Despite what they say, the atmosphere is not heavy. My theory about Otto Warmbier is that he got drunk and did something stupid. Imagine going to NK as being in a church; if you don't scream and misbehave, nothing happens. I can imagine, on large scale, people doing stupid things, and that likely will happen. But that's speculation on my part.
Definitely don't go with the Young Pioneer Tours (they were Warmbier's tour operator, and NK is not anymore in their destinations). They're a bunch of irresponsible, incompetent and dishonest people (I had my own misadventure with them). People like that should never be in charge of such delicate jobs.
edit: corrected the name of the travel agency