We are of opinion that instead of letting books grow moldy behind an iron grating, far from the vulgar gaze, it is better to let them wear out by being read.

This quote is suggesting that it is better for books to be read and used, rather than kept in a place where no one can see or access them, like behind a metal gate. The phrase “vulgar gaze” is referring to people who would be those who are not of high social class, or who are not viewed as being dignified or in the higher social circles. By keeping the books away from these people, it means that no one is able to benefit from the information or knowledge contained within the books. Therefore, it would be better to let the public have access to them, so that they can be read and used, rather than kept safe, but inaccessible.