Because libri is being talked about as a word form, not used to mean actual books.
In other words, the sentence is treating libri as a single linguistic item. That is why Italian can use singular è here.
Compare:
In the first case, you are talking about the word itself. In the second, you are talking about real books.
For the same reason: they are describing the word form as a grammatical item.
You can think of an implicit phrase such as:
So even though libri refers to more than one book in normal use, here it is being discussed as one word, and the description stays singular.
In italiano means in Italian or in the Italian language.
It sets the context for the statement, as if saying:
Putting it at the beginning is very natural.
Many do, especially masculine nouns, so this is a very useful rule.
Common pattern:
But not every noun follows it perfectly, and there are exceptions or irregular forms. For example:
So libro → libri is regular, but it is still important to learn exceptions as you go.