I was browsing through a copy of the World of Warcraft Strategy Guide for the Cataclysm expansion and looking at the book and the details it presented, I figured it would be a great way to lay out a campaign book for a gaming world.
The WoW Cataclysm book is four hundred pages of quests, stats and strategies. This could easily be translated into a campaign book. Yes, plenty of quests in the game are seriously dull 'go here and kill 'x' number of beasties' but even these could be fleshed out slightly. Let's say that you use 100 pages of the book for background information and the rest for adventures in the world at large. Let's say one page of description for a town or area, maybe a small map, and the next ten pages for quests and adventures, one adventure per page. The print in these books is quite small so you'd get quite a lot of detail in there, mayvbe even tiny stat blocks for the simpler systems.
So that's 400 pages including covers, so drop the actual usable pages to 390 for intros and other non-game info. 90 pages of world background, that's plenty, and then the last 300 pages for adventures and quests. 10 pages per location, so that's 30 locations with 9 adventures per location if we're looking at one adventure per page. That's 270 potential adventures, some of which can be linked to make longer campaigns. Let's say that the average gaming group game once per week and each adventure is designed for 1 session. That's 270 weeks of games, or roughly 5.2 years of adventuring.
Of course, somebody would have to write the damn thing. Yeesh.