As xyzmkrysvr said, no one seems to know, which is more than a little disappointing a month out from release. But I'd be on board with more Skyhold options!
It must be the case that Prima (and other guide writers) make agreements with the companies that make and produce these games whereby they get insider access from an early date. I'm not even talking about a month's worth of early access but being in on the process from beta and even alpha builds. I also imagine that the game company forks over a toolkit or similar data so that the guide writers can take a look at the internal mechanics. (That would be how such guides can give us so many behind-the-scenes stats, especially leveled lists.) Not to mention official art and perhaps screen images, especially for the hard copy of the guide. I can't even begin to guess at who pays whom how much or other specifics regarding such deals, but I do know that some official agreement has to happen between game makers and guide writers, otherwise the guides that make money would be in violation of copyright for commercializing another company's intellectual property. (This is why an ordinary fan/gamer can't just self-publish a guide using official images and make money off of it; fan-sites like wikis are legal so long as they don't make money.)
I have to admit that the bonus items got me to bite on buying the guide, but I did have other reasons as well. For one, it will almost certainly have artwork and possibly maps that are hard to reproduce without straight-up scanning them and thereby violating copyright. The wikis may (eventually) include an almost comprehensive amount of information, and they do sometimes correct official guides (for which I am very grateful), but they're entirely digital. Since the guide is both physical and digital, I can have the book open while using a wiki or using my computer for other things. So in my case I just had to decide whether the Prima guide was worth $25 or whatever it is online now, and the bonus items (despite the fact that they're still unknowns) just pushed me off the fence in the direction of yes (or more accurately, "why not?").
That said, I'm the sort of person who can't get enough information, whether it's playing games or anything else. I will read and watch everything I possibly can (within reason) before I start because it's much more enjoyable for me to have the feeling of knowing what I'm doing, where I'm going, and even what outcomes I want in advance. However, based on what you've said above, OP, you seem like the complete opposite. (Not that that's a bad thing, of course! Far from it.) I completely understand that desire to take the trial-and-error approach, but then the information in the guide might actually be spoilers for you. If that's the case, my advice would be to hold off on the guide (either buying it or just reading it) until you've had a chance to experiment with the game. If you're not certain the guide is worthwhile for you, you might also wait until reviews (and perhaps some excerpts) begin showing up online after it's released. Amazon, for example, might put up sample pages after the release date. Alternatively, you could have a look at Prima guides for other games you've played. That way you could get a sense of what the guides are like without spoiling yourself.
(Apologies for the massive walls of text!)