Information on local small businesses. The thing that immediately comes to mind is menus, store hours, etc. Just today I had to call to find out if a restaurant took reservations.
All those things on the edge that are actually the difference between success and failure, that wouldn't be so if everyone knew about them ...
Government information about legislation in preparation process
All proprietary know how, guidelines howto, real world experiences with solutions, approaches, process...what doesnt work at all and what is working fine and why.
In depth explanations of many topics are only available in books.
Information/data about our daily lives. More and more is being attempted to be collected, but the vast majority still is not.