James,
Like you I have many books on Gettysburg, probably 80-85 books.
I can say If I had to be limited to only ONE book on the battle, it would be this one. It is that broad ranging, and excellent production values in every respect.
Now a caveat. The way this material is presented it is indeed more of a compilation or compendium of available material rather than prospecting anything new, or making original interpretations of known preliminary or secondary material [because there aren't any footnotes/endnotes].
Indeed, what you are getting then is Mr. Adkin's words, none of which is footnoted nor sourced! Example, on unit strengths & casualties: I suspect if I checked Busey & Martin's 4th ed of Gettysburg Strength's & Losses, the numbers would be identical. But, Adkin's format for showing the data is so much better than the unformatted tables in the former.
Still, people have criticized this book for NOT attributing the source of the content properly [the assertions include not only writing but maps lifted from other works]. It is in my opinion, a legitimate criticism. And from an academic/literary sense, a serious one.
But for the average person, who appreciates high production values, lots of data presented graphically, maps, pictures, comprehensive information about the background of not only the campaign and battle but also tactics, weapons, organizations, drill et al and not so much on footnotes, its a good "one stop shopping" book to own for the casual enthusiast.
Keep in mind then when Adkins said such and such happened, or so and so
.you are left with his word only not knowing where he gained these insights. So with that you may just want to enjoy the production values & background information and get your scholarly interpretation elsewhere.
IMO, it is worth it to own for the former. I'm still keeping my other 80 or so books on the battle though so I can feel comfortable as to what happened when/why/how!