I purchased this book several years ago, precisely at the inception of my Outlander fixation. However, being my typical self, I never managed to actually read it until now.
Regarding companions, it is extremely detailed and encompasses nearly every piece of information one could desire to know (along with information that I truly had no inclination to know). I was most intrigued by the sections regarding how the characters and the story originated and indeed found those captivating. Of course, since I am reading this so many years later, a significant amount of the information I already knew from interviews and other sources (particularly the section covering the TV adaptation).
As for me, I had no interest in the sections that concentrated more on the 'technical' (for lack of a better term) aspects. Items such as pronunciation guides, family trees, and horoscopes held no appeal for me, so I either skimmed through or skipped them. I also skimmed over the research and writing guide/tips. I have no aspiration to ever become a historical fiction writer, and I also don't envision myself ever utilizing the bibliography. But it's great that it's there for anyone who wants it.
This book is definitely what I would classify as a collector's item. Something cool to possess but by no means essential. Apart from the very useful book summaries (which I have referred to previously and will无疑 do so again), I will not be rereading any part of this book.