Hi. You did not submit the pamphlet this question refers to, which makes it difficult for this question to be answered. However, I will try to help you as best I can.
As the National Congress is a political authority, made up of people who must be committed to the truth and accurate reporting of facts, Americans are likely to see a pamphlet published by Congress as a reliable source of information. The pamphlet can be considered credible if it is a political matter that has been drafted by a reputable and appropriate political group such as the national congress.
The words in the excerpt stated above which best show that Giblin views the Rosetta Stone as very important are the following: more valuable and famed. The adjective "more valuable" indicates value and worthy while "famed" indicates popularity or highly acclaimed.
