I checked *my* unloaned copy of FOTR, and in "A Conspiracy Unmasked" Merry says "I must be the only one in the Shire, besides you Frodo, that has seen the old fellow's secret book." While Merry can't absolutely say that he was the only one literally, the point is that if Sam had read it, Merry would certainly have known about it, and not have said what he did.

Sorry if I'm beating a dead horse here. :)