I have followed Paizo work on Pathfinder and Golarion from the beginning so I'm almost confident (and I also own) with all the books' contents. But I remember when I was attracted by Forgotten Realms but it seemed too vast for me not knowing well from where to start. I was unwilling to start any campaign there because I feared to be inadequate and missing something important.
Now the suggestion... A book that summarize the contents of what is out up to now, categorizing contents by regions, levels, suggesting how to prepare APs or campaign based on Pathfinder modules, how to improve those using pathfinder society adventures and, likely, also some third party scenarios or add ons. Where additional knowledge could be found on the explored regions for each suggested campaign, as pathfinder tales, companions, inner sea world guide, player guides...
Ok I think a book like this could be useful for me too...
Putting this in book form would make it obsolete somewhere between the time the manuscript went to print and the time it hit the shelves.
A continuously updated wiki handles it much better:
http://www.pathfinderwiki.com/wiki/Golarion
http://pathfinder.wikia.com/wiki/Golariopedia
Yeah, the old Wikia wiki is *way* out of date and quite a mess of ads and bad content to boot… you probably should just check out pathfinderwiki.com.