I believe "Our Dumb Century" was the only book with all original material that hadn't been on the website. That said, the "Ad Nauseum" compilation books are great for people like me who only read the website once in a while. Plus, I think they contain everything that was in the print edition too, which sometimes includes an extra article or editorial.
Also, I'm not sure how new you are to the Onion, but their Archive went paid subscriber-only a while ago, so a lot of the material from "Finest News Reporting" and "Dispatches From the Tenth Circle", as well as items from the older Ad Nauseums may not be available for free anymore.