The answer has to do with language, more so written than spoken.
Two places definitely had examples of it before 20CE.
It is an invention, in that it is something made up by people. It is not just a concept.
It has something to do with instructions (although perhaps not directly as much as I first thought).
It is often but not necessarily connected to translation and can be very useful in this.
It has something to do with a book or books.
It can often be found in a public library.
It is most often printed.
It often has different versions.
Spelling is an issue.
Transliteration is sometimes involved.
It is a group of words (kind of) but not a particular class of word.
Many instances of the answer start with the letter P.

It is not "a dictionary" and is not "something to do with an encyclopedia". It did not first appear in the Middle East and is not to do with one particular language.

The audience or some of the audience were impressed when the law, grammar or language schools were mentioned, although it is none of these things.

Oh, and the answer has more than one word (plus an indefinite article), not starting with the letter "P", and it does not have its own wikipedia entry.