Answer: Library books
There are several reasons why library books are the best source of information out of the given options. Your friends might not know the answers to a lot of your questions. Moreover, they are unlikely to be experts and therefore, more likely to make a mistake. The Internet, although full of useful and accurate information, also provides wrong information occasionally. Finally, Roman citizens might not be informed enough about the history of their city. On the other hand, books provide reliable, peer-reviewed information gathered by experts on the subject.