I recently watched the new film Truth in Numbers with great interest. I find the concern with "truth" to be a fascinating one, both in the title of the film and one of the comments made therein. I don't have a transcript of the film, but I recollect someone saying that Britannica never made any claims of truth. (If you caught the source and quote verbatim, please post it here.) As I claim in the book, you can find advertising claims by print-based reference works that are as hyperbolic as any Web 2.0 pundit today. Morton's The Story of Webster's Third discusses this when he talks about the critical and popular controversy associated with the publication of the Third in the 1960s. (And he argues that while the advertising department did go too far, those actually producing the dictionary never made such claims.) While I sadly do not have -- nor do I know if it exists -- a complete archive of reference work advertisements, this one for the famed 11th edition of Britannica seems germane: "Where uncertainty exists it applies a true standard of judgment; it corrects errors of fact. It is the authority" [original emphasis]. So much so, "It is the one library to which scientific men and great scholars go when they want to settle their doubts and their disputes." (Duke, Emergence of Advertising in America). Similarly, the 11th claimed to be "sum of all knowledge" a century before Wikipedia.
Of course, this is ancient history, I comment on a more recent advert from the 60s elsewhere.