If you haven't considered affiliate programs, that would be a good direction to go. You'll have to do more testing but once optimised you'll get a higher ROI than you would from most non-adsense ad companies.
They do have to be a little careful that the publishers aren't selling the same book on the same site (thus competing with their clients directly).
In which case they'd have to figure out how to look for "related" content to sell. However affiliate programs like Amazon's for one example definitely seems like a good suggestion.
In which case they'd have to figure out how to look for "related" content to sell. However affiliate programs like Amazon's for one example definitely seems like a good suggestion.